mx3+3nxy2-2x3-xy2+y中不含三次项
即mx3+3nxy2-2x3-xy2+y=mx3-2x3+3nxy2-xy2+y=(m-2)x3+(3n-1)xy2+y不含三次项
即m-2=0
3n-1=0
m=2
n=1/3
答：2m+3n=5