To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ r = 12 $ we have:
$$ 12 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 12 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 24 $$ $$ a = \dfrac{ 24 }{ \sqrt{ 3 } } $$ $$ a = 8 \sqrt{ 3 } $$