To find side $ a $ use formula:
$$ d = 2 \cdot a $$After substituting $d = \dfrac{ 51 }{ 2 }\, \text{cm}$ we have:
$$ \frac{ 51 }{ 2 }\, \text{cm} = 2 \cdot a $$ $$ a = \dfrac{ \frac{ 51 }{ 2 }\, \text{cm} }{ 2 } $$ $$ a = \frac{ 51 }{ 4 }\, \text{cm} $$