In this example question, it asks to give a systematic name for the following compound. In this compound, we have a benzene ring connected to two alkyl groups, which happen to be ethyl groups. They are both two-carbon chains. Here, you could start numbering at either carbon position on the benzene because they are identical. So, this could be 1, 2, 3, or you could go the other way, 1, 2, 3, 5, and 6. We have two ethyl groups in positions 1 and 3, so their numerical location is 1-3. There are two of them that are chemically indentical, so it's 'di,' and they are ethyl, and we end the name with benzene. Thus, the systematic name would be 1,3-Diethylbenzene.
Now, if they wanted us to provide the common name using ortho, meta, or para, since the groups are located at 1 and 3, the more common naming would be meta-diethylbenzene. Although the question does not ask for this, I'm mentioning it so that you can see the differences between the two naming systems. However, our official answer is 1,3-Diethylbenzene.
