Two parallel plate capacitors of capacity C and 3 C are connected in parallel combination and charged to a…

The charge on the capacitor is given by $q=CV$.

Initial charge on $C=18CV$

And initial charge on $3C=54CV$

Let final common potential difference $=V^{\text{'}}$

When a dielectric of dielectric constant $k$ is introduced between the plates of the capacitor the capacitance becomes $k$ times.

Applying conservation of charge,

$9C{V}^{\text{'}}+3C{V}^{\text{'}}=18CV+54CV$

$\Rightarrow 12C{V}^{\text{'}}=72CV\Rightarrow V^{\text{'}}=6 \mathrm{V}$