Final answer:
The slope, 0.32, means that the cost for renting the car on a particular day increases by 0.32 of a dollar for each mile she drives on that day.
Step-by-step explanation:
The slope of the equation C=0.32m+15 is 0.32. The slope represents the rate of change in the cost per day for each unit increase in the number of miles Janelle drives. In this case, for every additional mile Janelle drives, the cost of renting the car increases by 0.32 dollars.