5x+4y+z=5 (1)
4x+y-4z=15 (2)
x+y+z=0 (3)
(1)×4+(2)
24x+17y=35 (4)
(1)-(3)
4x+3y=5 (5)
(4)×3-(5)×17
72x-68x=105-85
4x=20
所以x=5
y=(5-4x)/3=-5
z=-x-y=0