(1) x(x-y)+y(x-y)=(x+y)(x-y)
(2) -12x^3+12x^2y-3xy^2=-3x(4x^2-4xy+y^2)=-3x(2x-y)^2
(3) (x+y)^2+m(x+y)=(x+y)(x+y+m)
(4) (x-a)(x+y)(ax+ay-bx+ab)
(5) (5x^2+4y^2)(5x^2-4y^2)
(6) 2^2+3(x-y)(4+3)=4+21(x-y)