5(x-y)+2(x-y)-4(x-y)
=（x-y）(5+2-4)
=3(x-y)
3(x-y)次方-4(x-y)+7(x-y)-6(x-y)次方
=(-4+7)(x-y)+(3-6)(x-y)次方
3(x-y)-3(x-y)次方