A regular n-gon is a polygon with n sides of equal length and with all the interior angles equal. Thus , for example a regular 3-gon is an equilateral triangle and a regular 6-gon is a regular hexagon. Hence in your problem I think n = 20 not 20ft. So you have a regular 20-gon.
I tried to sketch one but unless you make it very large it just looks like a circle.
My first thought was that a = 80 ft is the distance from the centre C of the 20-gon to one of its vertices as in the diagram. If you draw the circle with centre C and radius 80 ft its area is
π r2 = π 802 = 20,106 sq ft
which is just larger than the 20,000 sq ft area of the 20-gon, thus I think I was correct, a = 80 ft is the distance from the centre C of the 20-gon to one of its vertices.
Now can you find s, the length of a side of the 20-gon?