How do I write and balance this chemical equation as a double replacement reaction:

Question

K2SO4(aq) + PB(NO3)2 (aq)

Answer 1

Think about what double replacement means. Change partners.
K of one goes with NO3 of the other.
SO4 of one goes with Pb of the other. (By the way, Pb and PB aren't the same thing)

Answer 2

You have to recognize that SO4-- has a charge of -2 and NO3- has a charge of -1. That will tell you that the products must be KNO3 and PbSO4.
The balanced reaction is

K2SO4(aq) + Pb(NO3)2 (aq)
-> PbSO4 + 2 KNO3

If I remember my chemistry correctly, the lead sulfate (PbSO4) is insoluble in water and will precipitate out. An arrow pointing down is often written after precipitates.