Answer
The potential difference between $a$ and $b$ is $~~9.0~V$
Work Step by Step
To find the potential difference between $a$ and $b$, we can consider the potential difference across emfs $\mathscr{E}_2$ and $\mathscr{E}_3$:
$\Delta V = 5.0~V+4.0~V = 9.0~V$
The potential difference between $a$ and $b$ is $~~9.0~V$
