STEP 1: find side $ a $
To find side $ a $ use formula:
$$ d = 2 \cdot a $$After substituting $d = 30\, \text{cm}$ we have:
$$ 30\, \text{cm} = 2 \cdot a $$ $$ a = \dfrac{ 30\, \text{cm} }{ 2 } $$ $$ a = 15\, \text{cm} $$STEP 2:
The radius of the circumcircle is equal the side of the hexagon. $$ R = 15 $$