Both can be right because if the function goes up, turns and goes down, between x = -2 and x = 2, it can happen that f(-2) = f(2) and then the average rate of change is [f(2) - f(-2)] / [2-(-2)] which is 0/4 = 0.
The best definition for the term substitution would be...
the action of replacing someone or something with another person or thing.
If you are asking 7/8 - 1/x= 3/4,
first put 3/4 to 6/8 to make it easier.
Move the 7/8 over to the right. You will get 1/x=-1/8.
Simplify, it is x=-8.
Check=7/8 - 1/8=6/8
It checks out.
