What is the area of the octagon you can see here

Try to look at it this way:
What makes the shape?
You can divide it into 2 rectangles with the long side being 6.36 m • 2 + the short side being 9 m. Now you can find the area of these. Then you have 4 right angled triangles left. The area of a triangle is 1/2 • height • baseline which gives you 1/2 • 6.36 • 6.36.
Now just sum the area of the rectangles and the triangles.