We take values of f(0) and f(2), sum them up to find distance between them, than sum x values that are corespondent for f(0) and f(2) and we get average rate of change.
f(0) = -2
f(2) = 6
Avg = (2+6)/(0+2) = 8/2 = 4
Answer is 4.
Note that we had to use absolute value of f(0) because we need distance between those points and not mathematical sum of them...