STEP 1: find radius $ r $
To find radius $ r $ use formula:
$$ d = 2 \cdot r $$After substituting $d = 8\, \text{cm}$ we have:
$$ 8\, \text{cm} = 2 \cdot r $$ $$ r = \dfrac{ 8\, \text{cm} }{ 2 } $$ $$ r = 4\, \text{cm} $$STEP 2: find volume $ V $
To find volume $ V $ use formula:
$$ V = \dfrac{ h ^{ 2 } \cdot r \cdot \pi}{ 3 } $$After substituting $r = 4\, \text{cm}$ and $h = 4\, \text{cm}$ we have:
$$ V = \dfrac{ 16\, \text{cm}^2 \cdot 4\, \text{cm} \cdot \pi}{ 3 }$$$$ V = \dfrac{ 64\, \text{cm}^3 \, \pi}{ 3 } $$$$ V = \frac{ 64 }{ 3 }\pi\, \text{cm}^3 $$