First figure out what the products are for each equation, one of them is already give to you, so you just have to unlasso (figure out the charges) the ions. Once you figure out the products, make sure the same number of atoms is on the reactants side as the products side.
a) H2C2O4+2KOH⇒2H2O+K2C2O4
In this example C2O4 2- is one of the ions and K+ is the other ion, this yields K2C2O4. There have to be 2 K atoms on the products side, because there are 2 K atoms on the reactants side, so add a K to KOH. Then you have 2 extra Os and Hs on the reactants side, so you have to add a 2 in front of H2O.
b) CdBr2+Na2S⇒CdS+2NaBr
c) 3NaOH+Fe(NO3)3⇒Fe(OH)3+3NaNO3