Notice that there are two adults in each equation. That means the difference in the price each group paid is because of the difference in the number of students! So you can actually just find the difference between the two equations and solve for s, and you will be headed in the right direction!
Equation 2 - Equation 1: 0a + 2s = $18
s = $9
Now substitute s into either equation, and you'll find the adult price:
