otherclass.m

//Other class.h
//Include the needed method's class's .h
#import "AppDelegate.h"
@interface aWebView : UIViewController {
    
   //Create the delegate
    AppDelegate *appDelegate;
}

//OtherClass.m
//Call the method in the other class with the appDelegate
- (IBAction) klose:(id)sender{
    appDelegate = (SergeAppDelegate*)[[UIApplication sharedApplication] delegate];
    [appDelegate.mainViewController hideTheTabBar:NO];
}

viewcontroller.m

//ViewController.h
@interface MainViewController : UIViewController {

}
- (void)hideTheTabBar:(BOOL)hide;

//ViewController.m
- (void)hideTheTabBar:(BOOL)hide
{
    theTabBar.hidden = hide;
}