\item \self Consider the following formula.
$$\phi \coloneqq \forall y \big(P(x) \land Q(y)\big) \rightarrow  \exists x \big(R(y) \land Q(x)\big)$$
\begin{enumerate}
    \item  Compute $\phi[f(y)/x]$.
    \item  Compute $\phi[f(x)/y]$.
    \item  Compute $\phi[k/z]$.
    \item  Compute $\phi[x/z]$.
\end{enumerate}