(x^4乘x^3乘x^2)减(2x^4乘x^5加x^2乘x^7减3x^9)
=(x^4*x^3*x^2)-(2x^4*x^5+x^2*x^7-3x^9)
=(x^(2+3+4))-(2x^(4+5)+x^(2+7)-3x^9)
=x^9-(2x^9+x^9-3x^9)
=x^9-(3x^9-3x^9)
=x^9-0
=x^9