A car owner found that her monthly cost of driving her car depends on the number of miles she drives. She calculates that if she drives 320 miles per month, her monthly cost is $270. If she drives 400 miles per month, her monthly cost is $297. Assume that the monthly cost, C, of driving the car is a linear function of the number of miles, d, she drives in a month.
The formula for the monthly cost, C, of driving the car as a function of the number of miles, d, she drives in a month, is