It is given that:
[tex]s=-16t^2+144t[/tex]It is said to find the time when the rocket hits the ground and s is the height of rocket.
When the rocket hits the ground, s=0.
[tex]\begin{gathered} 0=-16t^2+144t \\ t(-16t+144)=0 \\ t=0,t=\frac{144}{16}=9 \end{gathered}[/tex]Hence time is either 0 or 9 seconds. Hence after the rocket is in the air it will hit the ground after 9 seconds.