Two parallel plate capacitors of capacity \( C \) and \( 3C \) are connected in parallel combination and charged to a potential difference \( 18 \, \text{V} \). The battery is then disconnected and the space between the plates of the capacitor of capacity \( C \) is completely filled with a material of dielectric constant \( 9 \). The final potential difference across the combination of capacitors will be __________ \( \text{V} \).
