$7 is what you'll pay even if you don't ride any rides at all. Each ride is an additional $2. "n" is the number of rides, our unknown. If we ride 4 rides, then n = 4. Filling in our equation accordingly, C(n) = 2(4)+7 which means that the cost for 4 rides is $15 (taking into account we are also paying admission. The rides alone only cost 2(4) which is $8).
