Charlie runs a book rental business. He currently charges $3 per book and rents out an average of 38 books a day.

According to a study, for every 50C increase in rental price, the average business can expect to lose 4 rentals a day.

Complete the equation that models this scenario, where b(x) is the revenue generated and x is the number of 50 price increases.

b(x)= x + x+ [