Hi mate

the company sells 7000(1-p/35) books, but the cost of producing is 1500+2.5x, where x means the amount of sold books

thus 1500+2.5*(7000)(1-p/35), because 7000(1-p/35) = x

The company's costs are 1500+2.5*(7000)(1-p/35) depending on p, p = price

The company sells 7000(1-p/35) books, gets p dollars for each sold book, that means it gets 7000(1-p/35)*p dollars

Your profit now is Earned Dollars - Costs

=> f(p) = (7000*(1 - p/35))*p - 7000*(1 - p/35)

You got to solve this:

=> f'(p) = ... (derive f)

f''(p) = 0 => p = ...

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i got p = $ 18