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}{2}=\dfrac{1}{3}+\dfrac{1}{R_2}
\\\\
6R_2\left(\dfrac{1}{2}\right)=\left(\dfrac{1}{3}+\dfrac{1}{R_2}\right)6R_2
\\\\
3R_2(1)=2R_2(1)+6(1)
\\\\
3R_2=2R_2+6
\\\\
3R_2-2R_2=6
\\\\
R_2=6
.\end{array}
Hence, the other resistance is $
6 \text{ ohms}
.$
