Hi there! The answer is 6 seconds.
We want to know when the rocket hits the ground, so when t = 0. Therefore, we must set up and solve an equation.
[tex] - 16t {}^{2} + 96t = 0[/tex]
[tex]16t ( - t + 6) = 0[/tex]
[tex]16t = 0 \: \: or \: \: - t + 6 = 0[/tex]
[tex]t = 0 \: \: \: \: \: \: or \: \: t = 6[/tex]
Therefore the answer is 6 seconds (the rocket has returned after 6 seconds is)