x = 3
2x + y - z = 3 → (1)
- x + y + 2z = 0 → (2)
3x + 2y + z = 9 → (3)
we require to eliminate the y and z terms from the equations
(1) + (3) : 5x + 3y = 12 → (4)
multiply (1) by 2
4x +2y - 2z = 6 → (5)
(2) + (5) : 3x + 3y = 6 → (6)
(4) - (6) : 2x = 6 ⇒ x = 3