9x+3y+z=0 (1)
x-y+z=0 (2)
z=-3 (3)
(1)+(2)*3
12x+4z=0 (4)
将（3）代入（4）得
12x+4*（-3）=0
x=1
将x=1,z=-3代入（2）得
1-y-3=0
y=-2