Please help me I have a calculus word problem?

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?

c. Find the maximum height,

d. At what time does the object return to the height of the cliff? What is the velocity at this time?

e. What is the velocity when the object reaches the ground (which is 20 m below the cliff)?

I need to use calculus to solve these and i have no idea how. Also, could you explain how you got the answers? Thanks.