Remember, that in order to add or subtract terms, they must be alike. In order for terms to be "alike", they must have the same exact variable attached to them. If those variables have exponents, then those exponents must be identical as well. For example, 2a and 5a are like terms because they have the same variables. 2a and 5b would NOT be alike. They have different variables.
BUT... What if negatives are involved?!?!? Watch the videos below to help explain what to do should you find yourself in this "negative" situation! (Get it? Ha ha ha!!!!!!!!)