6mx的平方+4nxy+2x+2xy-x的平方+y-4
=(6m-1)x^2+(4n+2)xy+2x+y-4
不含x平方和 xy项
6m-1=0,4n+2=0
6m=1,2n=-1
6m-2n+2=1-(-1)+2=4