$$\ce{C2O4^2- + MnO2 -> Mn^2+ + CO2}$$
I think that the half reactions are
$$\ce{C2O4^2- -> CO2}$$ $$\ce{MnO2 -> Mn^2+}$$
I am supposed to balance these by adding water, $\ce{H+}$ atoms and by adding $\ce{e-}$’s, but I’m just not sure on the method to do this as we’ve covered it extremely quickly.
First, I found the oxidation numbers for the overall equation, and I think that $\ce{C2O4^2-}$ is the reducing agent because $\ce{C}$ is losing charge from +3 to +4, I just don’t know how to use that to balance this. Any help would be appreciated.