if x-y=6 and x^2+y^2=24, find x^3-y^3
(x - y)^2 = x^2 - 2xy + y^2 => 2xy = x^2 + y^2 - (x - y)^2 => 2xy = 24 - 6^2 = -12 => xy = -6
x^3 - y^3 = (x - y) (x^2 + x y + y^2) = 6( 24 - 6 ) = 108
