The admission fee for an amusement park is $12 for adults and $6.50 for children. one weekend, 2904 people paid admission for the amusement park, and the park made $27,126. how many adults and how many children paid to go to the amusement park that weekend?

It seems that you have missed the necessary options for us to answer this question, but anyway, here is the answer.
Based on the given scenario above, the expression would be this: 
a + c = 2904
12a + 6.5c = 27,126
Wherein a is for adult, and c is for children. 
Here is the solution:
a + c = 2904
a = 2904 - c
Next,
12 (2904-c) + 6.5c = 27,126
34,848 - 12c + 6.5c = 27,126
-5.5c = 27,126 - 34, 848
-5.5c = - 7,722 (divide both sides by -5.5)
c = 1,404

Now going back to the first expression:
a + c = 2904
a + 1,404 = 2904
a = 2904 - 1,404
a = 1500

Hope this answer helps.