A car rental agency rents 420 cars per day at a rate of $20 …

A car rental agency rents 420 cars per day at a rate of $20 per day. For each ​$1 increase in​ rate, 10 fewer cars are rented. Find the function that represents the revenue. At what rate should the cars be rented to produce the maximum​ revenue?  The revenue function is R(x) = . The rate that will yield maximum revenue is .