y= -16t^2 +400, where t represents the number after the camera is dropped. How long will it take before Kay sees the splash from the camera hitting the water?

Your problem statement needs a bit of correction:

"Y= -16t^2 +400, where t represents the number OF SECONDS after the camera is dropped. How long will it BE before Kay sees the splash from the camera hitting the water?"

Set y = -16t^2 + 400 = 0.  We assume that 400 and 0 represent feet (English system).

Then 16t^2 = 400, and so 25 = t^2.  Only the positive square root makes sense here, and that root is 5 (seconds).