x+y+z=14 (1)
7z=x+y+2 (2)
x+z=y (3)
(3)代入(1)
y+y=14
y=7
x+z=7 (4)
7z=x+y+2=x+9 (5)
由(4)得x=7-z代入(5)
7z=7-z+9
8z=16
z=2
x=7-z=5
y=7