A 20 ft-long rod is cut into 12 pieces, which are welded together to form the frame

Question:

A 20 ft-long rod is cut into 12 pieces, which are welded together to form the frame of a rectangular box. The length of the box's base is 15 in. longer than its width.
(a) Create a polynomial expression for the volume V in terms ofx.
(b) Make a plot of V versus x.
(c) Determine the x that maximizes the volume and determine that volume.