a car rental company charges an initial fee plus an additional fee for each mile driven. the charge depends on the type of car: economy or luxury. the charge (in dollars) to rent an economy car is given by the function , where is the number of miles driven. the charge (in dollars) to rent a luxury car is given by the function . let be how much more it costs to rent a luxury car than an economy car (in dollars). write an equation relating to . simplify your answer as much as possible.