Question 153873
Let x = number of tickets ordered for children at $7.50 each
  x-8 = number of tickets ordered for adults at $12.00 each

Equation : 

7.5(x) + 12(x-8) = 138

7.5x + 12x - 96  = 138

      19.5x - 96 = 138
 
19.5x = 138 + 96

19.5x = 234

x = 234/19.5

x = 12  ( tickets ordered for children )

x-8 = 12-8 = 4 ( tickets ordered for adults )