The length of the skid mark left by a car when braking, in feet, can be used to predict the speed, in miles per hour, at which the car was driving when it hit the brakes. The speed of the car is given by the square root of 21 times the length of the skid mark. Use function notation to write this function rule.
Let l be the length of the skid mark and S(l) be the estimated speed of a car, in mph, that left a skid mark of l feet.
