Use your hint.

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"

Complete derivative:

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.