Note: It would help if you could post in a less general category and profide a more descriptive title. It might give a clue as to your level or what class you are in or what tools should be used.
In this case, it's a nice differential calculus problem. If you're in a different class, that will be a problem.
r and h are functions of time.
V is constant, so dV = 0
dr is given as -0.05"/sec
Find dh when r = 2.5"
dV is known.
r is given.
dh/dt is what we need. We'll have to solve for it after everything else is found.
h - You will have to solve for this from the Hint.
dr/dt is given.
Substitute and solve.
Additional Note: Please show your work. It is a great place to learn where you are in your mathematics understanding.