Average rate of change means find the slope of the secant line. So if there is a function f(x) and you want to find the average R.O.C over the interval [a,b], it would be (f(b)-f(a))/(b-a)
you know that r is less than or equal to -8, so you write that, and then you know that r is also greater than 0, so then you add that to it, and then you have the answer. r will be in the middle because it is the range of what are can be.