Let x = width and diameter;
y = height of the rectangle part.
Then the perimeter
P = ====> + = 12 ====> y = .
The area A = + = + = - - + = + -
Then the condition for the maximum area = 0 takes the form
= 0, or = 6 ====> x = = = 3.36 ft.
Answer. The maximum area is at x = 3.36 ft (the width),
y = = = 1.68 ft (the height).