I need to use calculus for this question. Im in calc 1.

You are standing on a planet on a cliff that is 20 m high. An object is thrown straight up over the egde of the cliff. If the height can be modeled as d(t)=-5.7t^2 + 36t + 20

a. Find the average velocity over the first three seconds

b. What is the speed after 2 seconds?

v(t) = d'(t) ... speed = |v(t)|
c. Find the maximum height,

max height will occur when v(t) = 0
d. At what time does the object return to the height of the cliff? What is the velocity at this time?

solve for d(t) = 20
e. What is the velocity when the object reaches the ground (which is 20 m below the cliff)?

find the time when d(t) = 0 , then find v(t) for that time.