First we note that we have a regular polygon with 9 sides. We can split the polygon in 9 triangles with a 40 degree angle each.
We know that the area of a triangle is 1/2 bh.
The base of each triangle is sin20=2x/8.65
And the height is cos20=x/8.65
Solving for the values we can plug them in the formula Area= 9 (1/2 bh)
So the area should be equal to 216.43 mm^2 rounded to the nearest tenth.