Well, you are looking for an expression for dh/dt...once you have this you can substitute 6 for h for your answer.

You have V in terms of r and h. First use the fact that the ratio of r to h stays constant to find a formula for V in terms of h only. Then you can calculate dV/dh.

You are given dV/dt, so no you can use the chain rule to find dh/dt.