Combining like terms is a topic that seems really easy to some students and a complete mystery to others. I talk about how each variable can be a different dollar bill amount. For example, x is a 1 dollar bill and y is a 5 dollar bill. If you are counting how much you have, you have to get all the ones that equal the same amount together. If you have a document camera, you could put a bunch of change up there and ask students what is the easiest way to count? Someone will eventually suggest separating them out into piles of pennies, nickel, dimes, and quarters. That is basically what we are doing when we combine like terms. It makes it easier. Anytime you can make math relate to money, it helps.
I also use food as an example if they are struggling. I say x is hamburgers and y is fries. You can’t count how many “hamburger fries” you have because they can’t be combined. This always makes student laugh and they will want to debate it for a while, but the idea is there.
I also draw a line, squiggly line, and an arrow under the different terms to help kids combine them correctly (shown on filled out graphic organizer). You could highlight with different colors or draw circles, squares, and triangles around them too. Just be careful that the sign in front of the term is included or they will confuse themselves.