if you have a regular octagon with a side 7, what is the area?
join the centre to each vertex,
that would give you 8 isosceles congruent triangles.
work on one of those.
the central angle is 45 degrees making each of the angles at the vertex 67.5 degrees.
You can find the height by
tan 37.5 = height/3.5
height = 8.44975
so the area of one such triangle is
(1/2)(7)(8.44975)
multiply by 8 to get the total area.
(I got 236.6, but check my arithmetic)