${{R}_{3,4}}={{R}_{3}}+{{R}_{4}}={{R}_{3}}+8$ $\frac{1}{{{R}_{eq}}}=\frac{1}{{{R}_{1}}}+\frac{1}{{{R}_{2}}}+\frac{1}{{{R}_{3,4}}}\Rightarrow \frac{1}{1/2}=\frac{1}{2}+\frac{1}{4}+\frac{1}{8+{{R}_{3}}}\Rightarrow \frac{10-6-3}{12}=\frac{1}{8+{{R}_{3}}}\Rightarrow {{R}_{3}}=4\Omega $