原式=x^4-3x^3+mx^2+nx^3-3nx^2+mnx+3x^2-9x+3m
=x^4+(-3+n)x^3+(m-3n+3)x^2+mnx-9x+3m
所以 -3+n=0,n=3
m-3n+3=0,m=6