To find side $ a $ use formula:

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

After substituting $V = 1102.5\, \text{cm}$ and $l = 21\, \text{cm}$ we have:

$$ 1102.5\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot \left( 21\, \text{cm} \right)^{4} }{ 4 } $$$$ 1102.5\, \text{cm} \cdot 4 = \sqrt{ 3 } \cdot a ^{ 2 } \cdot \left( 21\, \text{cm} \right)^{4} $$$$ 4410\, \text{cm} = \sqrt{ 3 } \cdot a ^{ 2 } \cdot \left( 21\, \text{cm} \right)^{4} $$$$ 4410\, \text{cm} = 21 \sqrt{ 3 }\, \text{cm} \cdot a ^{ 2 } $$$$ a ^{ 2 } = \dfrac{ 4410\, \text{cm}}{ 21 \sqrt{ 3 }\, \text{cm} } $$$$ a ^{ 2 } \approx 38.5929 $$$$ a \approx \sqrt{ 38.5929 } $$$$ a \approx 6.2123 $$

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