STEP 1:
The side of the hexagon is equal the radius of the circumcircle. $$ a = 12 $$
STEP 2: find diagonal $ d $
To find diagonal $ d $ use formula:
$$ d = 2 \cdot a $$After substituting $a = 12\, \text{cm}$ we have:
$$ d = 2 \cdot 12\, \text{cm} $$ $$ d = 24\, \text{cm} $$