The perimeter of the rectangle is 2(r+h)=12, so h=6-r.
Volume of cylinder is V=πr²(6-r)=6πr²-πr³, dV/dr=12πr-3πr²=0 at max or min. Divide through by 3π:
4r-r²=0=r(4-r), so r=0 is the minimum, and r=4 inches, therefore h=2 inches.
The unit of measurement in the question is inches, but the answer is expected to be in feet, so r=4/12=0.33ft and h=2/12=0.17ft. V=32π cubic inches=100.53 cubic inches or 0.06 cu ft.