Like terms have the same variable part — only the coefficients differ. They're the only terms you can combine.
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3x + 5x
Same variable x. Add coefficients: 8x.
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3x + 5y
Different variables — they don't combine. Final answer: 3x + 5y.
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2x²y + 3x²y − x²y
All have x²y. Combine: 4x²y.
x and x² are NOT like terms. Same letter, different power.