How deep is a water well if a rock dropped into it takes 2.13 seconds to splash down? My

How deep is a water well if a rock dropped into it takes 2.13 seconds to splash down? My teacher provided the equations average displacement = V * t and y= (Vi * t)1/2 * a * (t)^2. I’m not sure if I use one of these equations or if they don’t work with this problem.