x+y-z=0 (1)
2x+y+z=7 (2)
x-3y+z=8 (3)
(1)+(2)
3x+2y=7 (4)
(1)+(3)
2x-2y=8 (5)
(4)+(5)
5x=15
x=3
y=(2x-8)/2=-1
z=x+y=2
综上 x=3 y=-1 z=2