What is the oxidation number of carbon in methanal?

In the equation, $\mathrm{BiO}_3^{-}+6 \mathrm{H}^{+}+x \mathrm{e}^{-} \rightarrow \mathrm{Bi}^{3+}+3 \mathrm{H}_2 \mathrm{O}$ What is the value of $x$ ?

What is the charge required to convert $2 \mathrm{~mol} \mathrm{KMnO}_4$ to $\mathrm{MnSO}_4$ ?

What is oxidation number of sulphur in $\mathrm{SO}_3$ ?