If you draw a radius from the center to each corner of the pentagon you create 5 congruent isosceles triangles with a central(vertex) angle of 72 degrees and base angles of 54 degrees. the apothem forms a right triangle with each side with the hypotenuse being the radius , The apothem is the side opposite the 54 decree side and the adjacent side is 1/2 the side length
So sin54=6/r r=6/sin54 = 7.41
Cos54 =adj/6 adj =6/Cos54 = 10.2 so side length is 2 x 10.2 = 20.4
