Okay, here we have this:
Considering the provided information, we are going to calculate the requested rate of change, so we obtain the following:
We will replace in the rate of change formula with the following points: (1990, 1.2) and (1994, 1.6), then we have:
Rate of change=(f(b)-f(a))/(b-a)
Rate of change=(1.6-1.2)/(1994-1990)
Rate of change=0.4/4
Rate of change=0.1 Billion
Finally we obtain that the average rate of change in the number of public library visits from 1990 to 1994 is 0.1 billion.