Consider the expression - 2 + 5a + b + 2a - 5b.

Which term can be combined with 5a because they are like terms?

-2

b

2a

-5b

Like terms will have exactly the same variables

so 5a and 2a are like terms

examples:

3b and 4b are like terms

3b and 4a are not like terms

2a^2 + 3a^2 are like terms

2a and 3a^2 are not like terms

3ab and 4ba are also like terms ... even though the variables are switched

6b^2a and 6a^2b are not like terms

I agree with the first answer