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=0.50M+17.75 , 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=0.15M+3.90.

Let C be the total charge (in dollars) for a rental that includes insurance. Write an equation relating C to M. Simplify your answer as much as possible.