3x3-【x3+(6x2-7x)-2(x3-3xy-4y)]
=3x3-x3-(6x2-7x)+2(x3-3xy-4y)
=2x3-6x2+7x+2x3-6xy-8y
=4x3-6x2-6xy-y
=-4-6+6-1
=-5