Line 3 states $\sim\\!s$, i.e. that $s$ is _false_. Then use domination rule with $\rho=r$, and regard $s$ as $F$: $$s\land r \equiv F$$ which is just the 5th line [$\sim\\!(s\land r)$], and it came only from line 3.
The 6th line is derived from the 4th and 5th line by the law of Contraposition: if something of the form $A\to B$ is proven and $\sim\\!B$ is also known, then $\sim\\!A$ follows. Take $A=(\sim\\!p\,\lor\sim\\!q)$ and $B=(s\land r)$.