mx^3+3nxy^2-2x^3-xy^2+y =（mx^3-2x^3）+（3nxy^2-xy^2）+y =（m-2）x^3+（3n-1）xy^2+y 因为不含3次项,所以（m-2）x^3=0,所以m-2=0,所以m=2
