Seamingly easy movement?

I have a block that i’m trying to make move from right to left, and if the block gets to a certain point, it will move back to it’s original position.
So, i have this on my block (movieclip) rightnow

 onClipEvent(enterFrame) {
 	speed = 3
 	dist = 8.5
 	_x -= speed;
 	if (_x = dist) {
 		_x += 3

But all this is doing is throwing the block at the 8.5 distance…

any help??? Thanx in advance