To find radius $ r $ use formula:

$$ V = \dfrac{ 4 \cdot r^3 \cdot \pi }{ 3 }$$

After substituting $V = 36\pi\, \text{cm}$ we have:

$$ 36\pi\, \text{cm} = \dfrac{ 4 \cdot r^3 \cdot \pi }{ 3 }$$ $$ 36\pi\, \text{cm} \cdot 3 = 4 \cdot r^3 \cdot \pi $$ $$ 108\pi\, \text{cm} = 4 \cdot r^3 \cdot \pi $$ $$ r^3 \cdot \pi = \dfrac{ 108\pi\, \text{cm} }{ 4 } $$ $$ r^3 \cdot \pi = 27\pi\, \text{cm} $$ $$ r^3 = 27 $$ $$ r = \sqrt[3]{ 27 } $$ $$ r = 3 $$

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