Average velocity calculus

displacement, s metres, measured form a fixed point, 0, of a body moving in a straight line is given by s=5 +8t - 2t^2 where as t is measured in seconds

find the average velocity in the first 4 seconds

( the answer in the book says 4 m/s im not sure if its right..)

could someone explain how to do this? thanks

s(0) = 5
s(2) = 13 (maximum)
s(4) = 5
Average velocity for first 2 seconds = (13-5)/2 = 4 m/s
Average velocity for second 2 seconds = (5-13)/2 = -4 m/s
Average velocity for first seconds = (5-5)/2 = 0 m/s

v = d/t plugin 4 for t = displace = 5, time = 4 v=5/4