If x represents time, the average rate of change of the function f (x) in the first three seconds is ___.

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.