Question About Zooming

I have created a slider to control the width and height of a movieclip. However the numbers currently go the opposite way I would like them to go. currently if the slider is at the top that counts as 0 because of the y coordinate and the bottom counts as 200. what type of equation would I need to flip this so if the slider is at the top it’s 200 and at the bottom it is 0?

 
zoom_mc.slider_mc.onPress = function() {
 this.startDrag(false, 0, 0, 0, 192);
};
zoom_mc.slider_mc.onRelease = zoom_mc.slider_mc.onReleaseOutside=function () {
 this.stopDrag();
 container_mc._xscale = this._y;
 container_mc._yscale = this._y;
};

Thanks,
Saveth