We know that speed is a function of the distance traveled over the time. We are given 13 seconds for the time so we need to find the distance traveled in one rotation. This is also known as the circumference.
Circumference = pi(3.14) * d(2 * 8.1 meters) = 50.87 meters traveled
Finally, distance(50.87) / time(13 sec) = 3.91meters per second is the speed.
