The function below shows the number of car owners f(t), in thousands, in a city in different years t: f(t) = 0.25t^2 - 0.5t + 3.5 the average rate of change of f(t) from t = 2 to t = 6 is thousand owners per year

The average rate of change (m) is the ratio of the change in function value to the width of the interval:
  m = (f(6) - f(2))/(6 - 2)

To compute this, we need to compute f(6) and f(2).
  f(6) = (0.25*6 -0.5)*6 +3.5 = 9.5
  f(2) = (0.25*2 - 0.5)*2 +3.5 = 3.5
Then the average rate of change is
  m = (9.5 - 3.5)/(6 - 2) = 6/4 = 1.5

The average rate of change is 1.5 thousand owners per year.