English ain't my first language, so my explanation may be a little confusing.
Anyway, if I wanted to explain the limit simply, it's just the derivation of a number, meaning:
BUT! that's apply only when f(x) is ongoing when x=z. ( By ongoing meaning that for each x, from f(x) has one value.)
In your exercise, when x=-2, f(x) has 2 values f(......
