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