//  ActionScript 2.0

this.onMouseMove = function ( ) {
  begin_arrow._visible = false;
  var xpercent = _xmouse / Stage.width
  bg_mc._x = (Stage.width - bg_mc._width) * xpercent;
  updateAfterEvent ();									 
}

// _xmouse   stage.width    xpercent 
//   0          400           0
//   200        400           .5
//   400        400           1