Answer:
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).
Explanation: