A Rock Is Thrown Off a Cliff $s ( t ) = - 16 t ^ { 2 } + 16 t + 96$

Short Answer

A rock is thrown off a cliff. Its distance from the ground below at t seconds is $s ( t ) = - 16 t ^ { 2 } + 16 t + 96$ feet. When will it hit the ground?
Enter your answer exactly as: t = a. No units.