2 NaNO3 (aq) + 2 NaOH (aq) + Cr2O3 (s) → 2 Na2CrO4 (aq) + 2 NO (g) + H2O (l)
This is an oxidation-reduction (redox) reaction:
2 NV + 6 e- → 2 NII (reduction)
2 CrIII - 6 e- → 2 CrVI (oxidation)
NaNO3 is an oxidizing agent, Cr2O3 is a reducing agent.