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 E (in dollars) to rent an economy car is given by the function=E=0.60M+13.95,

where M is the number of miles driven. The charge L (in dollars) to rent a luxury car is given by the function=L=1.25M+21.40.

Let C be how much more it costs to rent a luxury car than an economy car (in dollars). Write an equation relating C to M. Simplify your answer as much as possible.