If 2x^2 -2y^2 = 72 and if x+y=18 what does x-y equal

First factor out the 2:

x^2 - y^2 = 36

x^2 - y^2 can be factored into (x+y)(y-y) so:

(x + y)(x - y) = 36

plug in your value:

18 (x-y) = 36 and you should be able to take it from there