The graph plots a function (x) if x represents time, the average rate of the function f(x) in the first three seconds

The rate of change of f(x) with respect to x is the slope of the line.

In the first 3 seconds, the line has the coordinates (0,100) and (3,220).
Therefore the slope is
(220 - 100)/(3 - 0) = 120/3 = 40 units/s.

40 units/s