18 seconds
Step-by-step explanation:
The object will reach the ground when its height h(t) = 0. Therefore, we can write
[tex]h(t) = 288t - 16t^2 = 0 \Rightarrow 16t^2 = 288t[/tex]
Solving for t, we get
[tex]16t = 288 \Rightarrow t = 18\:\text{s}[/tex]