To find side $ a $ use formula:
$$ R = \dfrac{ \sqrt{ 3 } \cdot a }{ 3 } $$After substituting $ R = 7 $ we have:
$$ 7 = \dfrac{ \sqrt{ 3 } \cdot a }{ 3 } $$ $$ \sqrt{ 3 } \cdot a = 7 \cdot 3 $$ $$ \sqrt{ 3 } \cdot a = 21 $$ $$ a = \dfrac{ 21 }{ \sqrt{ 3 } } $$ $$ a = 7 \sqrt{ 3 } $$