原式=6x²(x-y)-14y(x-y)²+2(x-y)³
=2(x-y)[3x²-7y(x-y)+(x-y)²]
=2(x-y)(3x²-7xy+7y²+x²-2xy+y²)
=2(x-y)(4x²-9xy+8y²)