There is no "rate of change on (−6,2) ".The average rate of change of a function f on an interval [a,b] is f(b)−f(a)b−a example, given a function like f(x)=x2+5 , we can find the average rate of change on [−6,2] by finding
f(2)−f(−6)2−(−6)=(9)−(41)8=−328=−4