The work done in bringing the rock from the ship to the ground is
the potential energy it has before Angel drops it.
Potential energy = (mass) x (gravity) x (height)
You have said that this amounts to 300 N-m .
300 N-m = (5 kg) x (gravity) x (10 m)
Divide each side by 10m : 30 N = (5 kg) x (gravity)
Divide each side by 5 kg : 6 N/kg = 6 m/s² = gravity
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By the way ... The rock doesn't do the work on the way down. Gravity does it !