"7-7-7 rule" is an old or dialect word meaning an fcc regulation stating that no single entity could own more than seven am radio stations, seven fm radio stations, and seven television stations nationwide at the same time. If your teen uses it, they have most likely picked it up from a book, video game, period drama, or a regional grandparent rather than a modern peer group. There is no cause for concern here; the word is largely out of contemporary teen usage. If it appears in a school text, it is being studied for vocabulary or historical context. You can treat it like any other archaic vocabulary item.