（-2y²）³+y乘y的5次方
=-8y^6+y^6=-7y^6
3x(3y-x)-(4x-3y)(x+3y)
=9xy-3x²-(4x²+12xy-3xy-9y²)
=9xy-3x²-4x²-9xy+9y²
=9y²-7x²