To find volume $ V $ use formula:
$$ V = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot l }{ 2 } $$After substituting $ a = 2.5 $ and $ l = 7.8 $ we have:
$$ V =\dfrac{ \sqrt{ 3 } \cdot 2.5 ^{ 2 } \cdot 7.8 }{ 2 } $$$$ V = \dfrac{ 10.8253 \cdot 7.8 }{ 2 }$$$$ V = \dfrac{ 84.4375 }{ 2 } $$$$ V = 42.2187 $$