因为不含三次,则
mx^3+3nxy^2-2x^3-xy^2+2x+y
=(m-2)x^3+(3n-1)xy^2+2x+y
m-2=0 3n-1=0
m=2 n=1/3
2m+3n=2*2+3*1/3=4+1=5