Explanation

(a) Equivalent resistance $R = 4 + \frac{3 \times 6}{3 + 6} = 6\Omega$ and main current $i = \frac{E}{R} = \frac{3}{6} = 0.5A$
Now potential difference across the combination of $3\Omega$ and $6\Omega,\ V = 0.5 \times \left( \frac{3 \times 6}{3 + 6} \right) = 1Volt$
The same potential difference also develops across $3\Omega$ resistance