To find side $ a $ use formula:
$$ A = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 }$$After substituting $ A = \frac{ 5 }{ 2 } $ we have:
$$ \frac{ 5 }{ 2 } = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 }$$ $$ \frac{ 5 }{ 2 } \cdot 4 = \sqrt{ 3 } \cdot a^2 $$ $$ 10 = \sqrt{ 3 } \cdot a^2 $$ $$ a^2 = \dfrac{ 10 }{ \sqrt{ 3 } } $$ $$ a^2 = \frac{ 10 \sqrt{ 3}}{ 3 } $$ $$ a = \sqrt{ \frac{ 10 \sqrt{ 3}}{ 3 } } $$$$ a \approx 2.4028 $$