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 +0.40M, 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 = 5.80 +0.25M.
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.