the average rate of change between two input values is the total change of the function values (output values) divided by the change in the input values.
so,
(h(7) - (h(3)) / (7 - 3)
h(7) = 7² - 7×7 + 6 = 49 - 49 + 6 = 6
h(3) = 3² - 7×3 + 6 = 9 - 21 + 6 = -6
7 - 3 = 4
(6 - -6)/4 = 12/4 = 3
the average change rate in that interval is 3 (or fully 3/1).
