x(x-1)-(x^2-y)=-2
x^2-x-x^2+y=-2
x-y=2
(x-y)^2=x^2+y^2-2xy=4
x^2+y^2=4+2xy
(x^2+y^2)/2-xy
=(4+2xy)/2-xy
=2+xy-xy
=2