Answer:
Multiply the circle's diameter by π (units do not matter).
Explanation:
A circle's diameter is the line that connects one point on the circle to the furthest point on the other side of the circle, passing through its centre. π is the ratio of the length of a circle's circumference to its diameter - it is therefore natural that multiplying a circle's diameter by π will give its circumference. π works, regardless of units used, so the fact the question is in feet does not matter!