Answer:
12 cm
Explanation:
If a regular hexagon is inscribed into a circle, the length of the side of a hexagon is the same as the radius.
You know this because a hexagon is made up of six equilateral triangles (all three sides are the same length). If inscribed in a circle, the common vertex (point of intersection for all triangles) is the center of the circle.
Attached is an image I found online that helps visualize what I am saying.