The slope is a constant rate of change. For one day at Cheap-o-Rentals, the cost is $39. So, that tells us that they have only given the cost of 1 day, which means as the days pass on, the slope changes along with it.
The cost per mile driven represents the slope of the function. So b. 15 cents per mile driven is the slope-the price increases as the number of miles increase.
