Question

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

Answer 1

2x^2-2y^2=72
divide both sides by 2
x^2-y^2=36
factor difference of 2 perfect squares
(x-y)(x+y)=36
given that x+y=18, subsitute 18 for x+y
(x-y)(18)=36
divide both sides by 18
x-y=2

Answer 2

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