(x+y)^2-6x^2+6y^2+9(x-y)^2
=x^2+2xy+y^2-6x^2+6y^2+9x^2-18xy+9y^2
=4x^2-16xy+16y^2
=4(x^2-4xy+4y^2)
=4(x-2y)^2