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 = 15.75+0.50 M, where M is the number of miles driven. The company also offers an option to ensure the car against damage. The insurance charge I in dollars is given by the function I = 5.70+0.15 M. Let C be the total cost in dollars for a rental that includes insurance. Write an equation relating C to M.