6mx^2+4nxy+2x+2xy-x^2+y+4
=（6m-1）x^2+(4n+2)xy+2x+y+4
不含二次项
所以二次项系数=0
即6m-1=4n+2=0
6m=1,4n=-2,2n=-1
所以6m-2n+2=1-(-1)+2=1+1+2=4