$${{R}_{n}}=\frac{{{R}_{1}}.({{R}_{2}}+{{R}_{3}})}{{{R}_{1}}+{{R}_{2}}+{{R}_{3}}}=4\Omega $$
$${{I}_{n}}=\frac{\xi }{r+{{R}_{n}}}=\frac{12}{1+4}=2,4A$$ $$\to {{U}_{1}}={{U}_{n}}={{I}_{n}}.{{R}_{n}}=2,4.4=9,6V$$
