To find side $ a $ use formula:
$$ V = \dfrac{ a ^{ 2 } \cdot h }{ 3 } $$After substituting $ V = 32 $ and $ h = 6 $ we have:
$$ 32 = \dfrac{ a ^{ 2 } \cdot 6 }{ 3 } $$$$ 32 \cdot 3 = a ^{ 2 } \cdot 6 $$$$ 96 = 6 \cdot a ^{ 2 } $$$$ a ^{ 2 } = \dfrac{ 96}{ 6 } $$$$ a ^{ 2 } = 16 $$$$ a = \sqrt{ 16 } $$$$ a = 4 $$