This is a word problem and I have no clue how to solve it.

Mt. Thor in Canada is said to have the longest vertical drop(4100 ft.) on Earth. How long would it take for a pebble dropped off the cliff to reach the ground? (Hint: The height of a falling object above the ground t seconds after it is dropped can be modeled by s(t) = -16t^2 + So where So is the initial height.)