how do you combine like terms

You can only add and subtract like terms.

Like terms have exactly the same letter and those letters have to be raised to exactly the same powers.

They do not have to be in the same order though     so  $$6ab^2 $ is a like term to $ 10b^2a$$

BUT   

$$13x^2,\;\;-3x^2y, \;\;-8xy^2,\;\;15x\;\;and\;\;4$$    are all unlike terms and none of them can not be added together. 

combining like terms is very simple it could be 2+2 which would of course equal 4 but do you know why we can add them together? Its because they are like terms aka they are the same as eachother and not just that but you could do 4+5 there still the same in a numerical sense. Where you cant do it is if you have 5+10x there not like terms so you cant add those together