If you're having trouble with redox, the way to do them is to split the two into half equations. You can either get the half equations from a data sheet or you can work them out yourself by:
1)Balancing elements
2)Balancing oxygen with H2O on the other side
3)Balancing the H's with H+ ions
4)Balancing the charge with electrons (e-)
That's for acid conditions.
So the half equation for SO3{-2} -> SO4{-2} would be:
1.SO3{-2} + H2O -> SO4{-2) (Balancing Oxygen)
2.SO3{-2} + H2O -> SO4{-2} + 4H{+} (Balancing hydrogen)
3.SO3{-2} + H2O -> SO4{-2} + 4H{+} + 4e- (Balancing charge)