Using the apothem, a, and the perimeter, p, the area, A, of a regular polygon is
[tex] A = \dfrac{pa}{2} [/tex]
You need to calculate the area of the octagon and subtract the area of the square.
The perimeter is 8 times the side.
[tex] A = \dfrac{pa}{2} - s^2 [/tex]
[tex] A = \dfrac{8 \times 13 \times 15.69}{2} - 13^2 [/tex]
[tex] A = 815.88 - 169 [/tex]
[tex] A = 646.88 [/tex]
