Help: motion?

var xSpeed:Number = 10;
var xEasy:Number = .6;
//var xStop:Number = 
ballX.onEnterFrame = function ():Void{
xSpeed *= xEasy;
this._x -= xSpeed;

I made this script to move my movie clip “ballX” to the left and stop and I want to return it back. How do I do it?
I want move the ballX few pixels to the left and then back to the right. I want to move let say 20 pix to the left and 20 pix back, I know how to do it with position of _x, but I want this “ballX” put it anywhere on the stage later on.