"Like terms" means the same kind of algebraic expression...the 6x and the 2x are "like terms" because they both have an x in them. The 9 and the 4 are both constants, which means they are plain numbers. So 6x and 2x is 8x .
9+4 is 13 just like elementary school.
So all together,
6x + 9 + 2x + 4
6x + 2x + 9 + 4
8x + 13
We can rearrange the terms because the problem has all addition in between the terms.
