Answer:
2, 6, and 7
Explanation:
3n is by itself because theres no other numbers that have the letter n.
6m and -11m are like terms bcs they both have the letter m (variable)
-3 and -18 are both like terms bcs they are both just numbers (or constants whatever)
so you would choose options 2, 6, and 7! hope this helps! :)