An Open-Top Box Is Constructed from a Square Piece of Cardboard

Multiple Choice

An open-top box is constructed from a square piece of cardboard with sides of length 9 ft. The volume of such a box is given by , where x is the height of the box. Find the volume of the box if its height is 4 ft. ​

A) 2.5 ft³
B) 3.5 ft³
C) 4 ft³
D) 6 ft³
E) 8.5 ft³