The displacement of the particle moving in a straight line as a function of time is s = t^2 - 8t + 19.
The average velocity over a time interval [a, b] is:
(b^2 - 8b + 19 - a^2 + 8a - 19)/(b - a)
=> (b^2 - a^2 - 8b + 8a)/(b - a)
=> ((b-a)(b +a) - 8(b - a))/(b - a)
=> a + b - 8
The instantaneous velocity at time t is 2t - 8. At t = 4 it is 0 m/s.
The average velocity over an interval [a, b] is a + b - 8 m/s and the instantaneous velocity at t = 4 is 0 m/s.