A new Car dealership has tracked the total number of car the graph below. What is the rate of change of the number of cars sold with
respect to the number of days?

Atourist boat is used for sightseeing in a nearby river. the boat travels 2.4 miles downstream and in the same amount of time, it travels 1.8 miles upstream. if the boat travels at an average speed of 21 miles per hour in the still water, find the current of the river.