Do you not know themeaningsof any of these words? If, for example, you know what "revenue function" means, this problem should be easy. If not where did you get this problem?

The crucial point is that "15 people" isn't it? Suppose there are 16 people. Then there is 1 "additional passenger" so everyone's fare is reduced by $3. That means every one's fare is now 500- 3= $497. The "revenue" is 497(16)= $7952. Suppose there are 17 people. Then there are 17- 15= 2 "additional passengers" so every one's fare reduced by 3(2)= $6, from $500 to 500- 6= $494. The revenue is 17(494).

What you want to do is go through that reasoning "in general" (which is, really, the whole point of algebra!). If there are "P" people, then there are P- 15 "additional passengers" so everyone's fare is reduced by 3(P- 15). That is, everyone pays 500- 3(P- 15). If P people each pay 500- 3(P- 15) dollars, what is the total revenue?