Please help/explain

3. If a rock is thrown upward on the planet Mars with a velocity of 10 m/'s, its height (in meters) after t seconds is given by H = 10t - 1.86t2 a) Find the velocity of the rock after 1 second b) Find the velocity of the rock when t = a c) When will the rock hit the surface of the planet? d) With what velocity will the rock hit the surface