To find radius $ r $ use formula:
$$ V = \dfrac{ r ^{ 2 } \cdot h \cdot \pi}{ 3 } $$After substituting $ V = 277.09 $ and $ h = 15 $ we have:
$$ 277.09 = \dfrac{ r ^{ 2 } \cdot 15 \cdot \pi}{ 3 } $$$$ 277.09 \cdot 3 = r ^{ 2 } \cdot 15 \cdot \pi $$$$ 831.27 = 15 \cdot r ^{ 2 } \cdot \pi $$$$ r ^{ 2 } = \dfrac{ 831.27}{ 15 \, \pi } $$$$ r ^{ 2 } = 17.6401 $$$$ r = \sqrt{ 17.6401 } $$$$ r = 4.2 $$