MainPage.xaml

<Page x:Class="LC_Points.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:Core="using:Microsoft.Xaml.Interactions.Core"
      xmlns:Interactivity="using:Microsoft.Xaml.Interactivity"
      xmlns:Series="using:WinRTXamlToolkit.Controls.DataVisualization.Charting"
      xmlns:charting="using:WinRTXamlToolkit.Controls.DataVisualization.Charting"
      xmlns:controls="using:WinRTXamlToolkit.Controls"
      xmlns:converter="using:LC_Points.Converter"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:datavis="using:WinRTXamlToolkit.Controls.DataVisualization"
      xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"
      xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
      xmlns:local="using:LC_Points"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
      mc:Ignorable="d">

    <Page.Resources>
        <converter:BoolToVisibilityConverter x:Key="BoolToVisibilityConverter" />
        <converter:BoolToNonVisibilityConverter x:Key="BoolToNonVisibilityConverter" />
    </Page.Resources>


    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="40*" />
            <RowDefinition Height="20*" />
            <RowDefinition Height="30*" />
            <RowDefinition Height="30*" />
            <RowDefinition Height="20*" />
            <RowDefinition Height="20*" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="4*" />
            <ColumnDefinition Width="3*" />
        </Grid.ColumnDefinitions>

        <StackPanel x:Name="TitlePanel"
                    Grid.Row="0"
                    Margin="12,17,0,28">
            <TextBlock Style="{StaticResource SubheaderTextBlockStyle}" Text="LC POINTS" />
            <TextBlock Margin="9,-7,0,0"
                       Foreground="DarkGreen"
                       Style="{StaticResource HeaderTextBlockStyle}"
                       Text="Home" />
        </StackPanel>
        <!--  TitlePanel contains the name of the application and page title  -->
        <ComboBox x:Name="subjectCmbBx"
                  Grid.Row="1"
                  Grid.ColumnSpan="2"
                  Width="199"
                  HorizontalAlignment="Left"
                  VerticalAlignment="Top"
                  DisplayMemberPath="Subject"
                  Header="Subjects"
                  ItemsSource="{Binding Subjects}"
                  PlaceholderText="Pick a subject"
                  SelectedItem="{Binding SelectedSubject,
                                         Mode=TwoWay}" />


        <ComboBox x:Name="ordinaryGradeCmbBx"
                  Grid.Row="1"
                  Grid.Column="0"
                  Grid.ColumnSpan="2"
                  Width="170"
                  HorizontalAlignment="Right"
                  DisplayMemberPath="Key"
                  Header="Grades"
                  ItemsSource="{Binding OrdinaryGradePointKV}"
                  PlaceholderText="Pick a grade"
                  SelectedValue="{Binding SelectedOrdinaryGrade,
                                          Mode=TwoWay}"
                  Visibility="{Binding IsHigher,
                                       Mode=TwoWay,
                                       Converter={StaticResource BoolToNonVisibilityConverter}}" />

        <ComboBox x:Name="higherGradeCmbBx"
                  Grid.Row="1"
                  Grid.Column="0"
                  Grid.ColumnSpan="2"
                  Width="170"
                  HorizontalAlignment="Right"
                  DisplayMemberPath="Key"
                  Header="Grades"
                  ItemsSource="{Binding HigherGradePointKV}"
                  PlaceholderText="Pick a grade"
                  SelectedValue="{Binding SelectedHigherGrade,
                                          Mode=TwoWay}"
                  Visibility="{Binding IsHigher,
                                       Mode=TwoWay,
                                       Converter={StaticResource BoolToVisibilityConverter}}" />

        //This button enable property is bound to the combo boxes being selected  ----->
        <Button x:Name="addGradeBtn"
                Grid.Row="2"
                HorizontalAlignment="Left"
                Command="{Binding Path=AddGradeCommand}"
                Content="Add Grade"
                IsEnabled="{Binding ButtonEnabled,
                                    Mode=TwoWay}" />

    </Grid>
</Page>

MainViewModel.cs

namespace LC_Points.ViewModel
{
    public class MainViewModel : ViewModelBase
    {
        private readonly IRepository<ScoreModel> _repository = App.ScoresRepository; 
     
        /// <summary>
        /// Initializes a new instance of the MainViewModel class.
        /// </summary>
        public MainViewModel()
        {
            //call methods to initilise list data
            InitSubjectTypes();
            InitOrdinaryGradePairs();
            InitHigherGradePairs();
        }

        public List<ScoreModel> Subjects { get; set; }
        public List<StringKeyValue> HigherGradePointKV { get; set; }
        public List<StringKeyValue> OrdinaryGradePointKV { get; set; }

        //Button enabled binding set based on the combo boxes being selected -->
        private ScoreModel _selectedSubject;
        public ScoreModel SelectedSubject
        {
            get { return _selectedSubject; }
            set
            {
                if (value != _selectedSubject)
                {
                    _selectedSubject = value;
                    RaisePropertyChanged("SelectedSubject");
                    ButtonEnabled = true;
                }
            }
        }

        private StringKeyValue _selectedHigherGrade;
        public StringKeyValue SelectedHigherGrade
        {
            get { return _selectedHigherGrade; }
            set
            {
                if (value != _selectedHigherGrade)
                {
                    _selectedHigherGrade = value;
                    RaisePropertyChanged("SelectedHigherGrade");
                    ButtonEnabled = true;
                }
                else
                {
                    ButtonEnabled = false;
                }
            }
        }

        private StringKeyValue _selectedOrdinaryGrade;
        public StringKeyValue SelectedOrdinaryGrade
        {
            get { return _selectedOrdinaryGrade; }
            set
            {
                if (value != _selectedOrdinaryGrade)
                {
                    _selectedOrdinaryGrade = value;
                    RaisePropertyChanged("SelectedOrdinaryGrade");
                    ButtonEnabled = true;

                }
                else
                {
                    ButtonEnabled = false;
                }
            }
        }
        
    }
}