6d and 3d are like terms, so you take them and add them together because they're both positive. You get 9d-e-2e. -e and -2e are also like terms so you would put these together but subtract them to get 9d-3e.
Like terms are terms that have the same variable parts.
In your problem, 6d and 3d are like terms and can be combined.
-e and -2e are like terms and can be combined.
6d - e + 3d - 2e =
= 6d + 3d - e - 2e
= 9d - 3e
Answer: 9d - 3e
