STEP 1:
The side of the hexagon is equal the radius of the circumcircle. $$ a = 20 $$
STEP 2: find area $ A $
To find area $ A $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 } $$After substituting $ a = 20 $ we have:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot 20^2 }{ 2 } $$ $$ A = \dfrac{ 3 \sqrt{ 3 } \cdot 400 }{ 2 }$$ $$ A = \dfrac{ 1200 \sqrt{ 3 } }{ 2 }$$ $$ A = 600 \sqrt{ 3 } $$