In regular math books, this is called "substituting" or "evaluating"... But, those are pretty big words for something so easy.
Check it out:
We are given something like this:
And we need to find what this thing equals when we put a number in for x... Like
x = 3
Everywhere you see an x... stick in a 3!
What about x = -5?
We need to be careful with the negative! Use ( )!
Let x = 2:
Let x = -1: