To find volume $ V $ use formula:
$$ V = \dfrac{ a ^{ 2 } \cdot h }{ 3 } $$After substituting $a = 18\, \text{cm}$ and $h = 20\, \text{cm}$ we have:
$$ V = \dfrac{ 324\, \text{cm}^2 \cdot 20\, \text{cm} }{ 3 }$$$$ V = \dfrac{ 6480\, \text{cm}^3 }{ 3 } $$$$ V = 2160\, \text{cm}^3 $$