finding Va-Vb

q₁+q₂=q

(law of conservation of charge says that not me) :P

the equivalent capacitor will store charge q

(again that is said be definition of equivalent capacitance .. not me) :P

find eq. capacitance... and q=CV

then find potential drop against 2μF capacitance and thus find potential difference across the the parallel setup.

Then find q₂ by making q₂=C_eq.V' (C_eq is equivalent capacitance for the 2 capacitor branch)

and V_a - q₂C_(4μF) = V_b

both has charge q by law of conservation of charge....

Now applying Kirchoff's Loop law..

12+q4-24+q2=0

gives, 3q4=12

q=16μC

V_a+q2=V_b

baki khud karo.. ! :P