To find side $ a $ use formula:
$$ R = \dfrac{ \sqrt{ 3 } \cdot a }{ 3 } $$After substituting $ R = 15 $ we have:
$$ 15 = \dfrac{ \sqrt{ 3 } \cdot a }{ 3 } $$ $$ \sqrt{ 3 } \cdot a = 15 \cdot 3 $$ $$ \sqrt{ 3 } \cdot a = 45 $$ $$ a = \dfrac{ 45 }{ \sqrt{ 3 } } $$ $$ a = 15 \sqrt{ 3 } $$