The registration period for the race is under way, and as the word is getting out, the number of participants registering is growing exponentially. The numbers from the first two days of registration are shown in the table. Opening Day of Registration 1 Day after Opening Day of Registration Participants Registered 15 21 Use the data in the table and the general form of an exponential growth equation, y = a(1 + r)x, to write an equation modeling the exponential growth of registrations, where x is the number of days since registration opened, a is the initial number of registrations, r is the rate of growth of registrations per day, and y is the total number of registrations.