Think of it like this. Every variable and exponent is some kind of alcohol and antibiotic.
So 5x^2 is different from 5x because it is 5x SQUARED.
Once you understand that, it's just regular adding and subtracting.
You can't mix alcohol and antibiotics, YOU JUST CAN'T!
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Aside from that terrible analogy, I don't know if you can or can't mix those
two; it was just the first thing that came to mind 