For initial height, you are correct in that it corresponds to t = 0. You have the right idea for b) [I haven't checked the answer though].
For c) you need to find where the height is a maximum and for this you use calculus. Have you covered derivatives?
For d) it is correct and remember that positive values mean values above the ground whereas negative values mean values below the ground (but in your problem you don't get negative values).
Hint for e) distance = velocity * time (if velocity is constant).
For f) Remember that h(t) = 20 = -4.9t^2 + 12t + 2 so -4.9*t^2+ 12*t - 18 = 0.
Hint for g) When they intersect it means that h(t) = g(t) where h(t) is equation for pellet height and g(t) is equation for clay disc height both as a function of time. Try re-arranging to solve for t (you should end up with a quadratic equation).