To find side $ a $ use formula:
$$ V = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot l }{ 2 } $$After substituting $ V = 5 $ and $ l = 5 $ we have:
$$ 5 = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot 5 }{ 2 } $$$$ 5 \cdot 2 = \sqrt{ 3 } \cdot a ^{ 2 } \cdot 5 $$$$ 10 = \sqrt{ 3 } \cdot a ^{ 2 } \cdot 5 $$$$ 10 = 5 \sqrt{ 3 } \cdot a ^{ 2 } $$$$ a ^{ 2 } = \dfrac{ 10}{ 5 \sqrt{ 3 } } $$$$ a ^{ 2 } = \frac{ 2 \sqrt{ 3}}{ 3 } $$$$ a = \sqrt{ \frac{ 2 \sqrt{ 3}}{ 3 } } $$$$ a = 1.0746 $$