How do you evaluate f(-2) given #f(x)=x^2-4#?
A function is a rule that tells you how you associate every input with its output.
This is what we mean when we write
In your example, you have
So, evaluating a function means exactly to do what I just said, but with a particular value chosen for
So, we only need to repeat the generic steps (square the input and subtract four), knowing that the input is
When we square it we get
Once you get this concept, there is a faster way to evaluate functions: simply substitute the generic