I'm confused on this concept

4.) A closed wooden box (top included) is to be constructed to have a volume of 48 cubic feet. The length of the box must be 6 times the height. Once it is built, it will be coated with a very expensive acrylic - so its surface area needs to be minimized. What dimensions should the box have in order to minimize the surface area? 5.) Use differentials to approximate the increase in volume of a sphere if the length of the radius of the sphere is changed from 9 cm to 9.1 cm. What is the exact change in volume