Type the correct answer in each box. Use numerals instead of words. If necessary, use / for the fraction bar(s). 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 50¢ 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.