Question

How do I know when to subtract or add combined like terms?

Answer 1

Only do addition and think of subtraction as adding a negative number. There is no subtraction, only addition and additive inverses. It's a useful mindset in this particular context, at least for me.

Answer 2

I will explain by giving example,
-3mn +7ab  - 8ab +6mn
First take like terms aside,
(- 3mn +6mn) +(7ab - 8ab )
I given bracket to understand.
First of all see the bigger number than see which sign the number has ,
In first bracket, 6 is bigger than 3 , so  substract.in second bracket 8 is bigger than 7 but has negative sign so here also substract but give bigger number sign after substracting.
3mn - 1ab
I think you have understood by the given example..
e.g  +8+6=14
-8 -6 = - 14
+8 -6 =2
- 8+6= - 2
Know better understood.