A car rental company's standard charge includes an initial fee plus an additional fee for each mile driven. The standard charge S (in dollars) is given by the function S = 17.75 +0.50M, where M is the number of miles driven. The company also offers an option to insure the car against damage. The insurance charge I (in dollars) is given by the function I =5.70 +0.25M. Let C be the total charge (in dollars) for a rental that includes Insurance. Write an equation relating Cto M. Simplify your answer as much as possible.​