4 times a number added to 8 times a number equals 36

Let a number be an arbitrary variable, let us call it x.

4 times a number:
4x

8 times a number:
8x

We add these together:
4x + 8x = 12x

We set 12x equal to 36 to get our final algebraic expression of:
12x = 36
If the problem asks you to solve for x, you type this algebraic expression into our search engine and get:
x = 3