·h = 126 ft. highest point rock reaches before it starts to fall
         |            [you don't need to know it is 126 ft. (vertex formula)]
cliff    |
---------·h = 100 ft  (cliff level)
         |
         |
         |  
         |
---------·h = 0 ft.  (water level)

Since the water level is 0, we set h=9 and solve for t:

h = -16t² + 60t + 100 
0 = -16t² + 60t + 100

divide both sides by -4

0 = 4t² - 15t - 25

0 = (2t+5)(t-5)

   0 = 2t+5;  0 = t-5
  -5 = 2t;    5 = t
-5/2 = t;

Ignore the negative answer.

Answer: 5 seconds.

Edwin