\(\overline{p\Rightarrow q} (r\iff s+t)\)
\(\overline{\overline p+ q} (\overline r+ s+t)(\overline {s+t}+r)\)
\(p \overline q (\overline r+ s+t)(\overline s \overline t+r)\)
\(p \overline q (\overline r \overline s \overline t+sr+tr)\)
\(p \overline q \overline r \overline s \overline t\)
\(\iff p\land \neg q \land \neg r\land \neg s\land \neg t\)