To find volume $ V $ use formula:
$$ V = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot l }{ 2 } $$After substituting $ a = 0.5 $ and $ l = 2.5 $ we have:
$$ V =\dfrac{ \sqrt{ 3 } \cdot 0.5 ^{ 2 } \cdot 2.5 }{ 2 } $$$$ V = \dfrac{ 0.433 \cdot 2.5 }{ 2 }$$$$ V = \dfrac{ 1.0825 }{ 2 } $$$$ V = 0.5413 $$