Rotation

i have movieclips that i assign y positions for. But not x. They just repel eachother, so i dont tell them to go to a set x position.

I want to make the movieclip rotate to so it points in what ever direction it is going. How can i do this without setting an x ?

i know how to do i using this,

var dx:Number = _xmouse - mc._x;
var dy:Number = _ymouse - mc._y;
var angle:Number = Math.atan2(dy, dx);

mc._rotation = angle * 180 / Math.PI;

but as i said, i dont know what its new x will be???