To find side $ a $ use formula:
$$ V = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot l }{ 2 } $$After substituting $ V = 5 $ and $ l = 2.5 $ we have:
$$ 5 = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot 2.5 }{ 2 } $$$$ 5 \cdot 2 = \sqrt{ 3 } \cdot a ^{ 2 } \cdot 2.5 $$$$ 10 = \sqrt{ 3 } \cdot a ^{ 2 } \cdot 2.5 $$$$ 10 = 4.3301 \cdot a ^{ 2 } $$$$ a ^{ 2 } = \dfrac{ 10}{ 4.3301 } $$$$ a ^{ 2 } = 2.3094 $$$$ a = \sqrt{ 2.3094 } $$$$ a = 1.5197 $$