public static function lEquation (x1:Float, x2:Float, y0:Float, y1:Float, y2:Float) :Float {
    return (x2 - x1) * (y0 - y1) / (y2 - y1) + x1;
}

What is this equation useful for? Anything that changes the values linearly.
>
Here's an example how to move a progress bar when you know the percent loaded:

progressBar.width = lEquation (0, 530, percent, 0, 100);

0 - 530 means that the width of the progressBar will change from 0 to 530 when the percent will take values from 0 to 100.
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Here's another example that will move a container on y axis by a scrollBar

var x1 = 0;
var x2 = x1 - container.height + containerMask.height;
var y0 = scrollBar.y;
var y1 = 0;
var y2 = y1 + containerMask.height - scrollBar.height;

container.y = lEquation (x1, x2, y0, y1, y2);

I find it useful because you don't have to think the formula again and again, you know visually what values to put in the function.

The math:

     ( x0 - x1 )   ( y0 - y1 )
     ----------- = -----------
     ( x2 - x1 )   ( y2 - y1 )

           (x2-x1)*(y0-y1)
     x0 = ----------------- +x1
               (y2-y1)