x²-5x+6=4
x²=5x-2
所以x³=x乘x²
=x(5x-2)
=5x²-2x
=5(5x-2)-2x
=23x-10
所以原式=(23x-10)-4(5x-2)-3x+2
=23x-10-20x+8-3x+2
=0