x + 2y + 3z = 13 2x + 2y + z =9 x + y + z =6

x+y+z = 6 (dikali 2)
2x+2y+2z = 12
2x+2y+z=9
----------------- (-)
z= 3 
sehingga x+y=3
x+2y+9=13
x+2y=4
.......
x+2y=4
x+y=3
------------ (-)
y=1

x= 2. 
jadi, x=2, y=1, z=3