Two parallel plate capacitors of capacity C and 3C are connected in...

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 $= {V}^{'}$

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

Applying conservation of charge,

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

$⇒ 12C{V}^{'} = 72CV ⇒ {V}^{'} = 6V$