A civil engineer wants to estimate the maximum number of cars that can safely travel on a particular road at a given speed. She assumes that each car is 18 feet long, travels at speed s, and follows the car in front of it at a safe distance for that speed. She finds that the number N of cars that can pass a given spot per minute is modeled by the function N(s)=89s?(18+18(s/21)^2) At what speed can the greatest number of cars travel safely on that road?