Actually Welcome to the Concept of the Average rate of change.
The average rate of change can be calculated by, finding the change of the two readings and getting a average between them.
here, for x =3 to x =5
1.) in the interval, x =3 to x = 4
the change is ==> 287 - 125 = 162
2.) in the interval, x = 4 to x=5
the change is ==> 549-287 = 262
Now the average ==> 162+262/2 ===> 212
Hence the average rate of change is 212