STEP 1:
The side of the hexagon is equal the radius of the circumcircle. $$ a = 700 $$
STEP 2: find diagonal $ s $
To find diagonal $ s $ use formula:
$$ s = \sqrt{ 3 } \cdot a $$After substituting $a = 700\, \text{cm}$ we have:
$$ s = \sqrt{ 3 } \cdot 700\, \text{cm} $$ $$ s = 700 \sqrt{ 3 }\, \text{cm} $$