Hi there!
We'll use the image below as a reference diagram. As you can see, the hexagon is divided into 6 triangles. And, one red line (center to point on the hexagon) would be the radius. Since we know that all of the triangles are equilateral triangles, and the hexagon is a regular hexagon, we know that one side of the hexagon is equal to 77m.
(If the circle actually has a diameter (all the way across the circle), the answer would be 38.5m)
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!