To find radius $ r $ use formula:
$$ V = \dfrac{ r ^{ 2 } \cdot h \cdot \pi}{ 3 } $$After substituting $ V = 803.84 $ and $ h = 12 $ we have:
$$ 803.84 = \dfrac{ r ^{ 2 } \cdot 12 \cdot \pi}{ 3 } $$$$ 803.84 \cdot 3 = r ^{ 2 } \cdot 12 \cdot \pi $$$$ 2411.52 = 12 \cdot r ^{ 2 } \cdot \pi $$$$ r ^{ 2 } = \dfrac{ 2411.52}{ 12 \, \pi } $$$$ r ^{ 2 } = 63.9676 $$$$ r = \sqrt{ 63.9676 } $$$$ r = 7.998 $$