很高兴为你解答,答案如下：
(x-y)²-4(x+y)²=(x-y)²-[2(x+y)]²=[(x-y)-2(x+y)][(x-y)+2(x+y)]=(x-y-2x-2y)(x-y+2x+2y)=-(3x+y)(x+3y)