STEP 1: find side $ a $
To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $r = 7\, \text{cm}$ we have:
$$ 7\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 7\, \text{cm} \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 14\, \text{cm} $$ $$ a = \dfrac{ 14\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a = \frac{ 14 \sqrt{ 3}}{ 3 }\, \text{cm} $$STEP 2: find diagonal $ d $
To find diagonal $ d $ use formula:
$$ d = 2 \cdot a $$After substituting $a = \dfrac{ 14 \sqrt{ 3}}{ 3 }\, \text{cm}$ we have:
$$ d = 2 \cdot \frac{ 14 \sqrt{ 3}}{ 3 }\, \text{cm} $$ $$ d = \frac{ 28 \sqrt{ 3}}{ 3 }\, \text{cm} $$