9514 1404 393
Answer:
 4.12 seconds
Step-by-step explanation:
Use the given initial height for Ball 2 to fill in that value in the function.
 f(t) = -16t² +272
You want to find t when f(t) = 0.
 0 = -16t² +272
 0 = -t² +17 . . . . . . . divide by 16
 t² = 17 . . . . . . . . . add t²
 t = √17 ≈ 4.123 . . . . take the positive square root
Ball 2 reaches the ground after 4.12 seconds.