Nothing.A foot is a 1-dimensional measure of length and cannot be converted to square feet which is a 2-dimensional measure of area.
A foot is a measure of distance, while the square foot is a measure of area. The two units are, therefore, incompatible.
3,136,320 square feet
The area of 72 what? Square miles? Acres? Square Astronomical Units?
Divide 72 by 12 to get 6 ft. You divide by 12 because there are 12 inches in 1 foot.