5x + 4y = 12 (1)

3x - 3y = 18 divide through by 3 → x - y = 6 .....

multiply through by 4 → 4x - 4y = 24 (2)

Add (1) and (2)

9x = 36 divide through by 9

x = 4

Using x - y = 6, we have

4 - y = 6 add y to both sides, subtract 6 from both sides

4 - 6 = y = -2