To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ r = 5 $ we have:
$$ 5 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 5 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 10 $$ $$ a = \dfrac{ 10 }{ \sqrt{ 3 } } $$ $$ a = \frac{ 10 \sqrt{ 3}}{ 3 } $$