This value is approximate.
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Explanation:
You are correct in noticing that h = 0 corresponds when the rocket is on the ground.
Plug this h value into the equation to get [tex]0 = -16t^2+100t+50[/tex]
You'll need the quadratic formula to solve for t.
Plug in a = -16, b = 100, c = 50.
[tex]t = \frac{-b\pm\sqrt{b^2-4ac}}{2a}\\\\t = \frac{-(100)\pm\sqrt{(100)^2-4(-16)(50)}}{2(-16)}\\\\t = \frac{-100\pm\sqrt{13200}}{-32}\\\\t = \frac{-100+\sqrt{13200}}{-32} \ \text{ or } \ t = \frac{-100-\sqrt{13200}}{-32}\\\\t \approx -0.47 \ \text{ or } \ t \approx 6.72\\\\[/tex]
Ignore the negative solution because a negative time value does not make sense. The only reasonable solution is roughly t = 6.72
It takes approximately 6.72 seconds for the rocket to hit the ground.