9x + 1/2y = 51 7x + 1/3y = 39 use the elimination method
9x + 1/2y = 51 → 18x + y = 102 → -18x - y = - 102 (1)
7x + 1/3y = 39 → 21x + y = 117 → 21x + y = 117 (2)
Add (1) to (2) and we have
3x = 15 → x = 5
So
18(5) + y = 102
90 + y = 102 → y = 12