x^2-y^2=-40
x^2-2xy+y^2=16
(x-y)(x+y)=-40
(x-y)^2=16
a) x-y=4
x+y=-10
x=-3, y=-7
b) x-y=-4
x+y=10
x=3 y=7
(-3.-7) or (3,7)
