Revenue = (profit)(number sold)
Calculate the current revenue:
Profit: $99 - $63
Number sold: 23
Adjust the current revenue equation:
Let x = the number of times the price is decreased by $2.
R(x) = (36 − 2x)(23 + 5x)
Optimize (that is, use derivatives to find the optimal value of x). But note, the question is asking about price, not x.