x^2-y^2-4x+4
=(x-2)^2-y^2
=(x+y-2)(x-y-2)
(x+y)^2+4(x-y)^2-4(x^2-y^2)
=(x+y)^2-4(x+y)(x-y)+4(x-y)^2
=[(x+y)-2(x-y)]^2
=(3y-x)^2