To find side $ a $ use formula:

$$ V = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot l }{ 4 } $$

After substituting $V = 5\, \text{cm}$ and $l = 2.5\, \text{cm}$ we have:

$$ 5\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot \left( 2.5\, \text{cm} \right)^{4} }{ 4 } $$$$ 5\, \text{cm} \cdot 4 = \sqrt{ 3 } \cdot a ^{ 2 } \cdot \left( 2.5\, \text{cm} \right)^{4} $$$$ 20\, \text{cm} = \sqrt{ 3 } \cdot a ^{ 2 } \cdot \left( 2.5\, \text{cm} \right)^{4} $$$$ 20\, \text{cm} = 4.3301\, \text{cm} \cdot a ^{ 2 } $$$$ a ^{ 2 } = \dfrac{ 20\, \text{cm}}{ 4.3301\, \text{cm} } $$$$ a ^{ 2 } \approx 1.4702 $$$$ a \approx \sqrt{ 1.4702 } $$$$ a \approx 1.2125 $$

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