21 A closed box with a square base is to have a volume of 2400 cubic inches The material for the top and the bottom of the box is to cost 4 per square inch and the material for the sides is to cost 3 per square inch Express the total cost of the material as a function of the length x of the base 6 Use the function and your calculator to find the dimensions of the box such that the total 3 cost of the material is least Be sure to give the equation that you graphed and window values you used Round your answer to the nearest tenth