To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ r = 25 $ we have:
$$ 25 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 25 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 50 $$ $$ a = \dfrac{ 50 }{ \sqrt{ 3 } } $$ $$ a = \frac{ 50 \sqrt{ 3}}{ 3 } $$