We spent a lot of time learning how to simplify expressions, sometimes with variables, sometimes without variables and we use the order of operations and other strategies to help us do that. Next we're going to see how we can evaluate expressions. Evaluating expressions means that we're finding the value of the expression. So we're going to see expressions that have variables in them. But to evaluate it, we need to know what number the variable represents. So here are the steps that we used to do it. First we substitute, which means just to plug in the given number for the variable and then we can simplify the expression using the order of operations that we already learned. So here we will evaluate A plus ten for A equals four. So they give us the expression A plus ten to start with. So let's write that out separately so we can see it nice and clearly. And they say to evaluate that expression for A equals four. So they're telling us what number A represents. They're saying A is equal to four. So wherever we see A, we can substitute it for four. So now our expression becomes four plus ten instead of just A plus ten since now we know that A really is four. And look at what we have here just two numbers to combine together. Four plus ten gives us 14. Evaluate M over three for M equals 21. So our expression is m over three. Now we have a fraction, but remember, fractions really just represent division. So this is just a division problem. They tell us to evaluate it for M equals 21 so we plug 21 in for M. Now we have 21 over three, which is 21 divided by three, and simplifies to seven. Let’s do one more, evaluate two B for B equals negative five. Start with our expression two B. We know that B is equal to negative five so we're going to plug that negative five in for B. But remember, two B means that the two is being multiplied by B. So when we plug the negative five in, we need to show that two is being multiplied by negative five. So we can use parentheses to show multiplication. Here you could use a multiplication symbol if you wanted to. But usually in algebra we'll use parentheses. So now we have two times negative five, which becomes negative ten. So that's all there is to evaluating expressions. Whatever numbers the variables represent, you just plug that number in for the variable and then simplify the expression like we normally do with order of operations.