Which expression is equivalent to (x + y) (x - y) ?

We can't see any choices but it doesn't matter. That's the factorization of the difference of two squares. It's important to recognize either side when you see it; the usual next move is to replace it with the other side.

(x + y) (x - y) = x² - y²

Answer: x² - y²

X^2-y^2

you would get x^2+xy-xy+y^2

the xy and - xy cancel