Evaluating expressions like 5x² & ⅓(6)ˣ | 6th grade | Khan Academy

What I want you to do is evaluate the expression 5x squared when x is equal to 3.

Pause this video and have a go at that.

All right, well we just have to think about every place we see an x; we'll now replace it with a 3. So this is going to be equivalent to 5 times, instead of x squared, it's going to be 5 times 3 squared.

And we know, from order of operations, we do the exponents first. That's why I actually put a parenthesis around the 3 squared to just make that clear. And 3 squared is, of course, equal to 9. And 5 times 9 is equal to 45.

Let's do another example that's a little bit different. Let's say I have the expression 1/3 times 6 to the x power, and I want to evaluate it when x is equal to 2.

Pause the video again and see if you can work that out.

Well, once again, everywhere where we see an x, we'll replace that with a 2. So this is going to be the same thing as 1/3 times 6 squared, where we saw the x, we now replace that with a 2.

And so this is going to be equal to, we do the exponent first, order of operations, so it's going to be 1/3 times 6 squared is 36, and one third of 36 is equal to 12. And we're done.