What's [ (x-y) + 7] times [ (x-y) - 7]?

Step-by-step explanation:

Let z = x - y

The given expression translates into

(z + 7) (z - 7) which is going to give you the difference of squares.

z^2 - 49

Now you need to know what z^2 is

z = x - y

z^2 = x^2 - 2xy + y^2

So the answer becomes

x^2 - 2xy + y^2 - 49