Question

How do you combine like terms for `-7mn-2mn^2-2mn + 8`?

Answer 1

In this expression only the first `-7mn` and the third `-2mn` are like terms. They both have the same literary part `mn` to the same powers and they can be added together.
The second term `-2mn^2` has `mn` as well BUT `n` is raised to the power `2`!!!

Imagine that like terms are objects. You can add or subtract together the same objects (for example 5 apples+2 apples= 7 apples) while you CANNOT do it with different objects (for example: 3 apples + 5 chairs=???)

In your case you can now write:

`-7mn-2mn^2-2mn+8` as:
`(-7mn-2mn)-2mn^2+8`
`-9mn-2mn^2+8`