Answer:
 about 6.418 seconds
Step-by-step explanation:
You apparently want to find t when h=0:
 0 = -16t^2 +84t +120
 0 = 4t^2 -21t -30 . . . . . . divide by -4
 t = (-(-21 ±√((-21)² -4(4)(-30)))/(2(4)) = (21±√921)/8 . . . . only the positive time is of interest
 t = 2.625+√14.390625 ≈ 6.419 . . . . seconds
It takes about 6.42 seconds for the rock to hit the water.
