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.
If you need extra help, give me a problem and I will help you solve it.