x+2x-y+6y-8=0 x+2x+1-(y-6y+9)=0 (x+1)-(y-3)=0 (x+1)=(y-3) x+1=±(y-3) x+1=y-3 x-y=-4 x+1=-(y-3) x+y=2(舍) ∴x-y=-4