多项式mx*x*x+3nxy²+2x*x*x-xy²+y不含三次式
mx*x*x+3nxy²+2x*x*x-xy²+y
=(m+2)x^3+(3n-1)xy²+y
m+2=03n-1=0
m=-2n=1/3
2m+3n
=2*(-2)+3*1/3
=-3