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It takes 4.3 seconds for the rocket to return to earth.
The equation is:

where -9.8m/sec² is the acceleration due to gravity, v₀ is the initial velocity, and h₀ is the initial height. We will go from the assumption that the rocket is launched from the ground, so h₀=0, and we are told that the initial velocity, v₀, is 42. This gives us:

We will use the quadratic formula to solve this. The quadratic formula is:

Plugging in our information we have:

x=0 is when the rocket is launched; x=4.3 is when the rocket lands.
The answer to the third question is that the root of the chord is a Gb.