x+y+z=15 (1)
x+2y+z=16 (2)
2x+y+z=7 (3)
(1)代入(2)得
(x+y+z)+y=15
y=1
(1)代入(3)得
x+(x+y+z)=7
x=-8
则
-8+1+z=15
z=22