Your PDF does not make much sense. I suggest you post the WHOLE question with ALL relevant information, not just the bits you think are appropriate.
You are given that the volume of cylinder of radius r and height h is , that and that .
ONE way to do this is to replace h in the volume formula with that formula in terms of r:
and then replace "r" in that by its formula in terms of t.
Oh, wait- there is no "r" in that formula- the " " terms canceled out and we got V= 10, a constant! Well, what does that tell you about its derivative?