$\frac{1}{{{R_{PQ}}}} = \frac{1}{{(6 + 2)}} + \frac{1}{3} + \frac{1}{{(4 + 12)}}$

$==>$ ${R_{PQ}} = \frac{{48}}{{25}}\,\Omega $

Current between $P$ and $Q$; $i = 1.5\,A$

So, potential difference between $P$ and $Q$

${V_{PQ}} = 1.5 \times \frac{{48}}{{25}} = 2.88\,V$.