Reaction has to be balanced in acidic medium 'O' atoms are balanced by adding $$\mathrm{H}_{2} \mathrm{O}$$ and then $\mathrm{H}$-atom is balanced by adding $\mathrm{H}^{+}$ ions and charge is balanced by $e^{\ominus}$.

Oxidation: $$\left.\mathrm{SO}_{3}^{2-}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{SO}_{4}^{2-}+2 \mathrm{H}^{+}+2 e^{\ominus}\right] \times 3$$

Reduction: $$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+14 \mathrm{H}^{+}+6 e^{\ominus} \rightarrow 2 \mathrm{Cr}^{3+}+7 \mathrm{H}_{2} \mathrm{O}$$

$$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+3 \mathrm{SO}_{3}^{2-}+8 \mathrm{H}^{\oplus} \rightarrow 2 \mathrm{Cr}^{3+}+3 \mathrm{SO}_{4}^{2-}+4 \mathrm{H}_{2} \mathrm{O}$$

$\mathrm{a}=1$

$\mathrm{b}=3$

$\mathrm{c}=8$