Answer:
As2O3 → H3AsO4
As2O3 → 2H3AsO4 balance As
5H2O + As2O3 → 2H3AsO4 balance O by adding H2O to one side
5H2O + As2O3 → 2H3AsO4 + 4H+ balance H by adding H+ to one side
5H2O + As2O3 → 2H3AsO4 + 4 H+ + 4e- balance charge by adding electrons to one side
Now do the same for the other part of the reaction
NO3- → NO
NO3- → NO + 2H2O
4H+ + NO3- → NO + 2H2O
3e- + 4H+ + NO3- → NO + 2H2O
Now cancel the electrons by multiplying the first equation by 3 and the second equation by 4, then add them together .
3As2O3 + 4NO3- + 7H2O + 4H+ → 6H3AsO4 + 4NO