To find the area of any regular (regular means all sides are = and all angles are =) poylgon when you know the length of the apothem and the length of the side you can use the formula:
A = (1/2)ap where a = the length of the apothem and p = the perimeter of the polygon. You can also use n*s in place of p where n = the number of sides of the polygon and s = the length of the side.
A= (1/2)(16)(19*8) (An octagon has 8 sides.)
A = 1216 in^2