To find side $ a $ use formula:
$$ s = \sqrt{ 3 } \cdot a $$After substituting $s = \dfrac{ 5 }{ 16 }\, \text{cm}$ we have:
$$ \frac{ 5 }{ 16 }\, \text{cm} = \sqrt{ 3 } \cdot a $$ $$ a = \dfrac{ \frac{ 5 }{ 16 }\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a = \frac{ 5 \sqrt{ 3}}{ 48 }\, \text{cm} $$