The formula of a cylinder is as follows:
V = πr²h
(I'm going to use random numbers to help explain)
Volume = 401.92
Height = 8
So if you were to rewrite the equation it would be:
401.92 = π×r²×8
From here you can divide the height by the volume:
50.24 = π×r²
If they do not tell you to put it in terms of pi (π) then divide by 3.14 or whatever value they give you:
16 = r²
Then from here do the √ (square root) of 16
4 = r
And that is how you can find the radius.
Hope this helps!