To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ r = 15 $ we have:
$$ 15 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 15 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 30 $$ $$ a = \dfrac{ 30 }{ \sqrt{ 3 } } $$ $$ a = 10 \sqrt{ 3 } $$