I really need help on this problem.. I don't know how to do it. I've been trying and trying so could you please help me.. step by step.. thanks so much!

An object is dropped from an inital height of s feet. The object's height at any time ,t, in seconds, is given by h= -16t squared + s. How long does it take for an object dropped from 300 feet to hit the ground? Round your result to two decimal places.

Thanks in advance.. I really need this answer before tomorrow.. so preferably in a couple hours from this being posted..