First what you must do is see if there is a greatest common factor. As you will see in this particular problem, there is not, so therefore you must factor by grouping.
(x+2)(y-4) is definitely the correct answer because if you really think about it, x*y=xy, x*-4=-4x, 2*y=2y, and 2*-4=-8!
So, I definitely believe that the answer is unmistakably (x+2)(y-4)!
