Arock is thrown in the air from the edge of a seaside cliff. its height in feet is represented by f(x) = –16(x2 – 3x – 18), where x is the number of seconds since the rock was thrown. the height of the rock is 0 feet when it hits the water. how long does it take the rock to hit the water? how many seconds?

Set the given equation equal to zero (0), since that's the height of the rock when it hits the water:

0 = -16(x^2 - 3x - 18)  (Note:  Please write x^2 to denote "square of x.")

Solving this equation for x, we get:

0 = -16(x-6)(x+3), and x=-3 or x=6.  Since x represents time, discard the negative value x=-3.

The rock hits the water after 6 seconds.