Is there any way I can script a part's orientation to be the opposite of itself?

I am trying to make a bow, and have got everything down apart from the orientation, as the arrow is facing the opposite way whenever it is shot. It is done using a ray cast, and the arrow goes to the endpoint, basically (I can tell you exactly what I did if necessary). So I want to fix that by simple using some sort of arithmetic function that gets the orientation, finds it's opposite, and sets the orientation to the opposite. Or a simpler way if there is one.