Add the following terms
3mn,-5mn,8mn,-4mn .

Hint-The terms can be added only if they have the same variables associated with them(that is only like terms can be added)

Complete step-by-step answer:
So, we have to add 3mn,-5mn,8mn and -4mn in this question
So, since they are like terms and the terms have the same variable (mn) associated with them, so we can add them up
So, we get
3mn+(-5mn)+(8mn)+(-4mn)
So, on adding this up, we get
11mn-9mn
=2mn
So, the answer for this question is 2mn.

Note: When adding up the like terms, give importance to the sign associated with them and add them up accordingly