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6-23.

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.