Its height in feet is represented by f(x)=-16(x^2-6x-16),where x is the number of seconds since the rock is thrown .the height of the rock is 0 feet when it hits water. how long doesit take the rock to hit the water

here we have is the height represented in feet.

we need to at when height is zero what will be the time taken to hit the ground.

which means we need to find x when f(x)=0.

we need to use quadratic formula to find x. or factorisation.

if we factor out

where

where as time cannot be negative so x=8 is the time when height is zero.