To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ r = \frac{ 9 }{ 2 } $ we have:
$$ \frac{ 9 }{ 2 } = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = \frac{ 9 }{ 2 } \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 9 $$ $$ a = \dfrac{ 9 }{ \sqrt{ 3 } } $$ $$ a = 3 \sqrt{ 3 } $$