Explanation:
the average rate of change is the change of the functional values over a given interval relative to (= divided by) the interval length (the change of the input variable in that interval).
in short :
( f (interval upper limit) - f (interval lower limit) ) /
(interval upper limit - interval lower limit)
upper limit = 8
lower limit = 0
the functional values (y) at these points are 36 and 12.
so, we get
(36 - 12) / (8 - 0) = 24 / 8 = 3
or
3/1