Today was the first day of school and i'm in calculus right now and i do not know how to do this problem....

If a ball is thrown into the air with a velocity of 40 ft/s, its height in feet t seconds later is given by y=40t-16t^2.

(a) Find the average velocity for the time period beginning with t=2 and lasting:

(i) 0.5 second

(ii) 0.1 second

(iii) 0.05 second

(iv) 0.01 second

(b) Estimate the instantaneous velocity when t=2.

Please help!