原式=3(x-4y)(-x-4y)-2(x+2y)²
=-3(x-4y)(x+4y)-2(x²+4xy+4y²)
=-3(x²-16y²)-2x²-8xy-8y²
=-3x²+48y²-2x²-8xy-8y²
= -5x²-8xy+40y²