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We have an inscribed hexagon

This will be composed of  6 equailateral  triangles  with sides  of 16

The total area  of this hexagon  =  6* (1/2) 16^2 * sin (60°)  = 768 * √3/2  = 384√3  units^2    (1)

And the circle will have a radius of 16....so its area is  pi (16)^2 = 256 pi  units^2    (2)

So...the shaded area  =  (2)  - (1)  = [ 256 pi  - 384√3]   ≈  139.1  units^2