STEP 1: find side $ a $
To find side $ a $ use formula:
$$ s = \sqrt{ 3 } \cdot a $$After substituting $s = 3.5\, \text{cm}$ we have:
$$ 3.5\, \text{cm} = \sqrt{ 3 } \cdot a $$ $$ a = \dfrac{ 3.5\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a \approx 2.0207\, \text{cm} $$STEP 2:
The radius of the circumcircle is equal the side of the hexagon. $$ R = 2.0207 $$