To find side $ a $ use formula:
$$ R = \dfrac{ \sqrt{ 3 } \cdot a }{ 3 } $$After substituting $ R = 20 $ we have:
$$ 20 = \dfrac{ \sqrt{ 3 } \cdot a }{ 3 } $$ $$ \sqrt{ 3 } \cdot a = 20 \cdot 3 $$ $$ \sqrt{ 3 } \cdot a = 60 $$ $$ a = \dfrac{ 60 }{ \sqrt{ 3 } } $$ $$ a = 20 \sqrt{ 3 } $$