原式=(3x²+7xy+2y²)(x-2y)
=(3x+y)(x+2y)(x-2y)
=(3x+y)[x²-(2y)²]
=(3x+y)(x²-4y²)
=3x×x²-3x×4y²+y×x²-y×4y²
=3x³-12xy²+x²y-4y³