2mn - m + 7n - ___

Let's try to factor it, thinking of "FOIL":



Let's try 2m and n for the "Firsts"



Then to get the 7n for the "Inners" we have to put a +7 in the right of the first parentheses.



Finally to get the -m for the "LASTS" we must put  in the remaining blank.



So when you FOIL that out you get



So the term we have to add is the "OUTERS" .

[You already had a - sign, so what we'd put in the blank would be . 

Edwin