(x-y)^2-4(x-y+3)
=(x-y)^2-4(x-y)+2^2-16
=(x-y-2)^2-16
=(x-y-2-4)(x-y-2+4)
=(x-y-6)(x-y+2)
回答的有点慢,手机不好使.
希望可以采纳