a
 Equivalent resistance \(R = 4 + \frac{{3 \times 6}}{{3 + 6}} = 6\,\Omega \) and main current \(i = \frac{E}{R} = \frac{3}{6} = 0.5\,A\)

Now potential difference across the combination of \(3\) \(\Omega\) and \(6\) \(\Omega\), \(V = 0.5 \times \left( {\frac{{3 \times 6}}{{3 + 6}}} \right) = 1\,Volt\)

The same potential difference, also develops across \(3\) \(\Omega\) resistance.