import flash.events.Event;

var nameObejct:String;
var velocity:Number;

var startPos:Number = Background.x;

stage.addEventListener(Event.ENTER_FRAME, moveBackground);
function moveBackground(e:Event):void {
    velocity = 10;

    Background.x -= velocity;
    if(Background.x <= 100) {
        Background.x = startPos;
    }
}