To find height $ h $ use formula:
$$ V = \dfrac{ r ^{ 2 } \cdot h \cdot \pi}{ 3 } $$After substituting $ V = 18 $ and $ r = 6 $ we have:
$$ 18 = \dfrac{ 6 ^{ 2 } \cdot h \cdot \pi}{ 3 } $$$$ 18 \cdot 3 = 6 ^{ 2 } \cdot h \cdot \pi $$$$ 54 = 36 \cdot h \cdot \pi $$$$ h = \dfrac{ 54 }{ 36 \, \pi} $$$$ h = 0.4775 $$