X+y+z=-6 X-2y+z=3 -2y+y+z=9

X+y+z=-6
X-2y+z=3
-------------- -
3y = 3
y = 3/3 = 1

-2y+y+z=9
-2(1) + 1 + z = 9
-2 + 1 + z = 9
1 + z = 9
z = 9 - 1 = 8

x + y + z = 6
x + 1 + 8 = 6
x + 9 = 6
x = 6 -9 = -3

maka,
X = -3
Y = 1
Z = 8