If (x + y)^2 = 78$ and $xy = 13$, what is $(x - y)^2
(x+y)^2=x^2+y^2+2xy.
x^2+y^2+2(13)=78, x^2+y^2=52.
(x-y)^2=x^2+y^2-2ab, so 52-2(13)=52-26=26-answer.
