To find side $ a $ use formula:
$$ R = \dfrac{ \sqrt{ 3 } \cdot a }{ 3 } $$After substituting $R = 9\, \text{cm}$ we have:
$$ 9\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a }{ 3 } $$ $$ \sqrt{ 3 } \cdot a = 9\, \text{cm} \cdot 3 $$ $$ \sqrt{ 3 } \cdot a = 27\, \text{cm} $$ $$ a = \dfrac{ 27\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a = 9 \sqrt{ 3 }\, \text{cm} $$