To find volume $ V $ use formula:
$$ V = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot l }{ 2 } $$After substituting $ a = 12 $ and $ l = 20 $ we have:
$$ V =\dfrac{ \sqrt{ 3 } \cdot 12 ^{ 2 } \cdot 20 }{ 2 } $$$$ V = \dfrac{ 144 \sqrt{ 3 } \cdot 20 }{ 2 }$$$$ V = \dfrac{ 2880 \sqrt{ 3 } }{ 2 } $$$$ V = 1440 \sqrt{ 3 } $$