Hot nitric acid oxidises arsenic(III) sulfide to arsenic acid and sulfate ions, and is itself…

Step 1. Electrons lost. Each arsenic goes from $+3$ to $+5$: a rise of 2, and there are two, so 4. Each sulfur goes from $-2$ to $+6$: a rise of 8, and there are three, so 24. One $\text{As}_2\text{S}_3$ loses $4 + 24 = 28$ electrons.

Step 2. Electrons gained. Each nitrogen goes from $+5$ to $+2$, a gain of 3.

Step 3. Match. The smallest number that 28 and 3 divide into is 84. So 3 $\text{As}_2\text{S}_3$ and 28 $\text{NO}_3^-$: $3\text{As}_2\text{S}_3 + 28\text{NO}_3^- \rightarrow 6\text{H}_3\text{AsO}_4 + 9\text{SO}_4^{2-} + 28\text{NO}$.

Step 4. Charge. The left side is $-28$ and the right side is $-18$. Add 10 $\text{H}^+$ on the left.

Step 5. Hydrogen. The right side has 18 hydrogen atoms and the left has 10. Add 4 water molecules on the left. Oxygen: $84 + 4 = 88$ on the left, and $24 + 36 + 28 = 88$ on the right.