This is a linear function and the rate of change is the same everywhere. Average rate of change = Δ f ( x ) / Δ x Δ f ( x ) - change of f ( x ),  Δ x - change of x. Average rate of change : ( 0 - 150 ) / ( 3 - 0 ) = - 150 / 3 = - 50   The average rate of change of the function f ( x ) in the first 3 seconds is - 50.