3x³-【x³+(6x²-7x)-2(x³-3xy-4y)】
=3x³-(x³+6x²-7x-2x³+6xy+8y)
=3x³-(-x³+4x²-7x+6xy+8y)
=3x³-x³-4x²+7x-6xy-8y
=2x³-4x²+7x-6xy-8y
=2*(-1)-4*1+7*1-6*(-1)*1-8*1
=-2-4+7+6-8
=-1