1.) are the terms that follow like terms?
3x and x
yes
no
-
2.) 3x2 and 3x
yes
no
-
3.) 3xy and 3yx
yes
no
-
4.) -m and m
yes
no
-
5.) 3xy2 and 3y2x
yes
no
-
6.) 3m and 3n
yes
no

1) yes (3x-x = 2x for example)
2) no (only 3x^2 is squared and they can't be summed or subtracted).
3) yes (they're just in different positions)
4) yes (they are +m and - m and 1m-1m = 0m)
5) yes (in both terms only y is squared)
6) no (different letters)