6mx²+4nxy+2x+2xy-x²+y+4
=(6m-1)x²+(4n+2)xy+2x+y+4
前两项是二次项
所以他们的系数是0
6m-1=0.4n+2=0
6m=1,2n=-1
6m-2n+2=1+1+2=4