To find side $ a $ use formula:
$$ s = \sqrt{ 3 } \cdot a $$After substituting $ s = \frac{ 63 }{ 100 } $ we have:
$$ \frac{ 63 }{ 100 } = \sqrt{ 3 } \cdot a $$ $$ a = \dfrac{ \frac{ 63 }{ 100 } }{ \sqrt{ 3 } } $$ $$ a = \frac{ 21 \sqrt{ 3}}{ 100 } $$