The radius of a merry-go-round is 8.1 m, and it takes 13 s to make a complete revolution. what is the direction of the momentum of this atom?

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. 

Iam looking for this answer too!