The cost y (in dollars) to rent a car is proportional to the number x of days that you rent the car. it costs $207 to rent the cars for 3 days. write an equation that represents the situation.

The volume of a spherical hot air balloon v(r) = 4 3 ? r3 changes as its radius changes. the radius is a function of time given by r(t) = 3t. find the instantaneous rate of change of the volume with respect to t at t = 2.