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Polygons don't have radii... the distance from the center to the midpoint of any side is called an apothem.
21.4
15 ft.
0.5*apothegm*perimeter = 0.5*3*20 = 30 sq feet
Area of regular polygon: 0.5*3*20 = 30 square feet
The circumference of a circle is the same as the perimeter of a circle, so the perimeter and circumference would both be 3 ft.