Let A = adult and s = student.
Multiply the cost of an adult ticket by number of adults: 13a.
Do the same for students: 9s
Add those together to get total money made:
13a + 9s = $2130
Then add the number of each tickets sold to get total sold:
a +s = 210 tickets.
13a + 9s = $2130, a +s = 210
The last answer is correct.