Now that we've seen the basics of algebraic expressions, remember that this letter x here is variable is a letter that's used to represent any number. It could be 3, negative 2, or even 0. The idea here is that just like with numbers, we're going to have to use operations, we're going to have to add, subtract, multiply, and divide variables when we're given their exact values. So the idea here is that some problems will actually tell us what x is equal to, and it'll ask us to calculate something. Like, it'll say x is equal to 3, and this is called evaluating an algebraic expression. When you evaluate an expression, what you're going to do is you're going to plug in those given values that the problem is telling you. You're going to plug those in for the variables, and then we're just going to use our order of operations. We're just going to use, first, remember, PEMDAS, and I've got it here just in case you need a little bit of a refresher. So the idea here is that 2x+5, that x could mean anything. But in this particular problem, it's telling you that x=3. So all we have to do is wherever we see an x, we just replace it and plug in a 3. That's what evaluating an expression means. Let's just jump right into this problem here. This 2x+5 if x is equal to 3, then I'm just going to replace the x with a 3. So this is 2 parenthesis 3 plus 5. You're usually going to see when you plug in stuff or when you plug in numbers for variables, they get this little parenthesis here. And so that's all there is to it. This x is equal to 3. Now we just have to use our order of operations. So we have to deal with parentheses first, and, technically, there's a parenthesis here, but there's just one number inside of it, so it kind of just goes away. There's no exponents. And then we've got multiplication and addition. So first, we have to multiply and divide before we add and subtract. And so when we multiply this 2 and the 3, this becomes 6, and then 6 +5 is equal to 11. Alright? So this is how you evaluate an algebraic expression. Everywhere you see an x, just replace it with a 3. Alright? So let's move on to the second part problem, now part b, which is a little bit more complicated. So we have negative and then we have 2(8-x) / 4x, but the idea is the same. Everywhere we see an x, we just replace it with a 3. So this just becomes, negative 28 minus 3 divided by 4 and then parenthesis 3. Notice how this x just became a 3, and this x on the bottom also just became a 3. Now we just have to use our order of operations. So first, we have to deal with the parentheses first. So we have negative, and this is going to be 2. Then we have 8 minus 3. This just becomes a 5. And then on the bottom, we're not going to do anything yet. This is going to be 4 times 3. And now we just have a bunch of multiple and division. There is no addition and subtraction in this problem. So all we have to do is we just go left to right and then top down. Right? So 2 times 5 is just equal to, don't forget the minus sign, negative 10 or sorry, 10 over here. And on the bottom, we have 4 times 3, and this just becomes 12. Alright? So all we have to do here is just now simplify the fraction. We've seen how to do this before. This is negative 5 over 6, and this is the answer to your problem. Alright? So that's what it means to evaluate an algebraic expression. Let me know if you have any questions, and thank you for watching.