To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ r = 18 $ we have:
$$ 18 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 18 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 36 $$ $$ a = \dfrac{ 36 }{ \sqrt{ 3 } } $$ $$ a = 12 \sqrt{ 3 } $$