x-y-3z=4 (1)
3x+y-z=6 (2)
-x+3y+z=8 (3)
(1)+(2)
4x-4z=10(4)
(1)×3+(3)
2x-8z=12+8=20
x-4z=10 (5)
(4)-(5)
3x=0
x=0
z=(x-10)/4=-5/2
y=x-3z-4=7/2