1.2x的三次方-4x的三次方:
2x^3-4x^3
=-2x^3
2.m的二次方(-m的三次方):
m^2*(-m^3)
=-m^5
3.(-x)的三次方×(-x的二次方)×x
=-x^3*(-x^2))x
=x^6