I’m guessing you mean like this. 2x+4y+5x+8y. 2x and 5x are like terms because the x’s are alike. If you add 2+5, the answer is 7. So therefore 2x+5x=7x.
4y and 8y are like terms because the y’s are alike. So the answer to 4y+8y would be 12y.
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