To find radius $ r $ use formula:
$$ V = \dfrac{ r ^{ 2 } \cdot h \cdot \pi}{ 3 } $$After substituting $ V = 75.36 $ and $ h = 8 $ we have:
$$ 75.36 = \dfrac{ r ^{ 2 } \cdot 8 \cdot \pi}{ 3 } $$$$ 75.36 \cdot 3 = r ^{ 2 } \cdot 8 \cdot \pi $$$$ 226.08 = 8 \cdot r ^{ 2 } \cdot \pi $$$$ r ^{ 2 } = \dfrac{ 226.08}{ 8 \, \pi } $$$$ r ^{ 2 } = 8.9954 $$$$ r = \sqrt{ 8.9954 } $$$$ r = 2.9992 $$