13x + 7y − 2x + 6a
The only like terms in this expression are `13x` and `-2x`. We cannot do anything with the `7y` or `6a`.
So we group together the terms we can subtract, and just leave the rest:
(13x − 2x) + 6a + 7y
= 6a + 11x + 7y
We usually present our variables in alphabetical order, but it is not essential.