4x^2+y^2+4xy+6x+3y-28
=(2x+y)^2+3(2x+y)-28
=(2x+y+7)(2x+y-4)
（x+y）^2-4（x^2-y^2）-21（x-y）^2
=（x+y）^2-4(x+y)(x-y)-21（x-y）^2
=[(x+y)-7(x-y)][(x+y)+3(x-y)]
=(x+y-7x+7y)(x+y+3x-3y)
=(-6x+8y)(4x-2y)
=4(-3x+4y)(2x-y)[(x+y)-7(x-y)][(x+y)+3(x-y)]这一部我看不懂十字交叉因式分解1 -7×1 3