i need help with calc

If a rock is thrown upward on the planet Mars with a velocity of 17 m/s, its height above the ground (in meters) after t seconds is given by H = 17t - 1.86t-. (a) Find the velocity (in m/s) of the rock after 2 seconds. m/s (b) Find the velocity (in m/s) of the rock when t = a. m/s (c) When (in seconds) will the rock hit the surface? (Round your answer to one decimal place.) t = S (d) With what velocity (in m/s) will the rock hit the surface? m/s