Rip-Off Rentals charges $25 per day plus 50¢ per mile to rent a mid-sized car. Your teacher will rent you his or her family sedan and charge you only 3¢ if you drive one mile, 6¢ if you drive two miles, 12¢ if you drive three, 24¢ for four, and so on. Homework Help ✎ Write an equation that will give you the cost to rent each car.

Don't rent your teacher's car; that's just asking for trouble.  And teachers, don't rent cars to students.

Let d be the number of days the car is rented for and m the number of miles it was driven while rented.

R.O.R. car costs in $:

R = 25d + 0.50m

Teacher car costs in $:

T = 0.03 · 2^m

where the caret ^ of course means exponentiation.