Generally, a / symbol is used to notate an ascending slide, while a \ symbol is used to notate a descending slide. So, 7/9\7 indicates sliding from the seventh fret, up to the ninth fret, and back to the seventh fret. If no number precedes the slide symbol, this indicates sliding from an indiscriminate fret.
It is also not uncommon to see the letter s used to notate a slide. This is somewhat less concise, as when sliding from an indiscriminate point (e.g. s9), it is unclear whether to slide up to the note, or down to the note.
