I am assuming that when you entered x1/2 you meant x^(1/2) which is √x.
I futher assume that the 2 at the end meant ².
-2(√x·y)² = -2(√x·y)·(√x·y) -2·(√x·√x)·(y·y) (Since you can rearrange when you multiply)
-2 · x · y² = -2xy²
If my assumptions are wrong, please repost.