What does “/” (forward slash) mean in this context

Question

I was reading A New Introduction to Modal Logic, written by G.E. Hughes and M.J. Cresswell, book on modal logic and the author uses the forward slash {/}:

but I was unable to understand it.

So I ask: what does / mean in this situation?

Answer 1

From context, it seems like $(3)[p \land q / p]$ means "Take statement (3), but substitute $p \land q$ for $p$."