解
mx立方+3nxy平方-2x立方+xy平方+y
=(m-2)x立方+(3n+1)xy平方+y
不含三次项
∴m-2=0
3n+1=0
∴m=2,n=-1/3
∴2m-3n
=2×2+3×(1/3)
=4+1
=5