*a number*means an arbitrary variable. We can pick any letter a-z except for i and e.

Let's choose x.

Four times a number:

4x

nine more than four times a numbrer

4x + 9

The phrase

*is*means equal to. We set 4x + 9 equal to 25 as our algebraic expression:

**4x + 9 = 25**

If the problem asks you to solve for x, we type it in our math solver and get:

x =

**4**