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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:

x^2 - 4x + 1

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!

x^2 - 4x + 1 = (3)^2 - 4(3) + 1 = -2

What about x = -5?
We need to be careful with the negative!  Use
(  )!

x^2 - 4x + 1 = (-5)^2 - 4(-5) + 1 = 46


TRY IT:

Let x = 2:

Let x = -1:


1