X=
area of an isosceles triangle=
area of polygon=

Area of the triangle =
We solve for half of one side
half-side^2 = 18.4^2  -17^2
half-side^2 = 338.56 -289
half-side^2 = 49.56
half-side = 7.0398863627
area of polygon = 17 * 7.0398863627 * 8 =
957.4245453298
(This matches the double-check below).


Double-Check
n = number of sides
polygon area = apothem^2 * n * tan (180/n)
polygon area = 17*17 * 8 * tan (22.5)
polygon area = 17*17 * 8 * 0.41421
polygon area = 957.65352

Source:
http://www.1728.org/polygon.htm