a + c = 342...a = 342 - c
12a + 5c = 2550
12(342 - c) + 5c = 2550
4104 - 12c + 5c = 2550
-12c + 5c = 2550 - 4104
-7c = - 1554
c = -1554/-7
c = 222
a + c = 342
a + 222 = 342
a = 342 - 222
a = 120
adult tickets sold (a) was 120, child tickets sold (c) was 222