What resistance must be placed in parallel with $12 \Omega$ to obtain a combined resistance of $4 \Omega$ ?
$$
\begin{array}{lc}
\text { From } & \frac{1}{R_{\mathrm{eq}}} & =\frac{1}{R_{1}}+\frac{1}{R_{2}} \\
\text { we have } & \frac{1}{4 \Omega} & =\frac{1}{12 \Omega}+\frac{1}{R_{2}} \\
\text { and so } & R_{2}=6 \Omega
\end{array}
$$