2x+2y+z＝5×3 6x+6y+3z＝15+2x-6y+4z＝-148x+7z＝18x+7z＝1×324x+21z=311x+3z=8×777x+21z=5677x+21z=56-24x+21z=353x=53x=1x=1代入 24x+21z=3 z=-1z=-1代6x+6y+3z＝15y=2