Answer
$6 \text{ ohms}$
Work Step by Step
Using the given formula, $
\dfrac{1}{R}=\dfrac{1}{R_1}+\dfrac{1}{R_2}
,$ then
\begin{array}{l}\require{cancel}
\dfrac{1}{R}=\dfrac{1}{12}+\dfrac{1}{12}
\\\\
\dfrac{1}{R}=\dfrac{2}{12}
\\\\
\dfrac{1}{R}=\dfrac{1}{6}
\\\\
1(6)=1(R)
\\\\
6=R
.\end{array}
Hence, the combined resistance is $
6 \text{ ohms}
.$
