Given: $C_1=12\ \mu F$ charged to $150$ V, connected to uncharged $C_2=6\ \mu F$.

Initial charge: $Q=C_1V=12\times150=1800\ \mu C$ (total charge conserved after connecting).

Effective capacitance (parallel combination after connecting): $C_1+C_2=18\ \mu F$

Final common potential difference:

$$V'=\frac{Q}{C_1+C_2}=\frac{1800}{18}=100\text{ V}$$

Charge on each capacitor:

$$Q_1'=C_1V'=12\times100=1200\ \mu C,\qquad Q_2'=C_2V'=6\times100=600\ \mu C$$