The plates of a parallel plate capacitor have charges Q and -2Q. It`s capacitance is C. The potential difference between it`s plates will be?

Just compare this with the case where Q AND -Q on plates of capacitor... here charge stored inside will be half of Q-(-Q). Which is egaual to Q again.... similarly if Q1 and Q2 are charges on plates, charge stored will be 1/2 ( Q1-Q2).  ...... SO IN THIS CASE CHARGE STORED IS 3/2 Q... AND THUS POTENTIAL DIFFERENCE WILL BE (3/2 Q) / C..... hope this would help you!!!