(-2x)(3x-x^2-mx^3+x^3+2mx^2)=-6x^2+2x^3+2mx^4-2x^4-4mx^3=-6x^2+（2-4m)x^3+2mx^4-2x^4因为要不含有x的三次所以2-4m=0即m=1/2
