# Defining Variables - Problem 2

To evaluate a variable expression, substitute in the given value of the variable and simplify. To evaluate an expression means to find the value of the expression.

Usually in Math variables are represented by the letter x but sometimes you get letters other than x. In this situation, we're going to be using the variable f. So this problem tells us evaluate f minus 4 when, we have to do when f is equal to 3 and we also have to do when f is equal -2. These are two different problems.

So what we call this in Math is substitution. What I'm going to be doing is taking my original problem, I'm going to rewrite it except for that where there used to be the letter f, I'm going to substitute in this number 3. So instead of using f right there, I'm going to erase it and write the number 3. Then evaluate means just do the Math 3 minus 4 equals -1. That's how substitution and evaluation works.

Here's another one. So I'm going to take my original expression f minus 4, only they told me f is the same thing as -2. So I'm going to substitute in -2. Now comes the evaluation part which means just calculate, do the math -2 minus 4 is equal to -6.

So those are two important instructions you're going to see in your Math homework, evaluate and substitute. Substitute means take the letter and replace it with the number they gave you and then evaluate means do the calculations, do the Math. So you guys can do it. Don't be freaked out by finding instructions.

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