x-y-z=-1 ①3x+5y+7z=11 ②4x-y+2z=-1 ③②-①*3 8y+10z=14 4y+5z=7 ④③-①*4 3y+6z=3 y+2z=1 ⑤⑤*4-④ 3z=-3 z=-1y=3x=1
