[(p ∨ q) =⇒ r] =⇒ (q ∧ r)
I got stuck after using implication and demorgan laws.
We do not know the system names you are to use. Therefore you must fill in the justifications.
$\begin{gathered}
(p \vee q) \Rightarrow \;r \hfill \\
\neg (p \vee \;q) \vee \;r \hfill \\
(\neg p \wedge \neg q) \vee \;r \hfill \\
(\neg p \vee r) \wedge (\neg q \vee r) \hfill \\
(\neg q \vee r) \hfill \\
q \Rightarrow \;r \hfill \\
\hfill \\
\end{gathered}$