We can get information out of a function too. Not sure this is the clearest example, but you can re-write it.

a function with a return.pde

float double7;

void setup() {
    size(512, 512);    
}

void draw(){

    double7 = double7er(mouseX);
    println("double7: " + double7);
    // double7 = (mouseX * 2) + 7;
    // println("double7: " + double7);
}


float double7er(float inputnumber){
    return (inputnumber * 2) + 7;
}