2x乘（-x)4次方-(-x)乘x的4次方
=2x(-x)^4-(-x)x^4
=2x^5+x^5
=3x^5
X乘X的M-1次方+X的平方乘X的M-2次方-3乘X的3次方乘X的M-3次方
=x*x^(m-1)+x^2*x^(m-2)-3*x^3*x^(m-3)
=x^m+x^m-3x^m
=-x^m