Here's how to write the balanced equation for the reaction of mercury(II) nitrate and sodium sulfide:

1. Write the reactants and products:

* Reactants: Mercury(II) nitrate (Hg(NO₃)₂) and sodium sulfide (Na₂S)

* Products: Mercury(II) sulfide (HgS) and sodium nitrate (NaNO₃)

2. Write the unbalanced equation:

Hg(NO₃)₂ (aq) + Na₂S (aq) → HgS (s) + NaNO₃ (aq)

3. Balance the equation:

* Mercury (Hg): Already balanced (1 on each side)

* Nitrate (NO₃): 2 on the left, 1 on the right. Add a coefficient of 2 in front of NaNO₃

* Sodium (Na): 2 on the left, 2 on the right (already balanced after balancing nitrate)

* Sulfide (S): Already balanced (1 on each side)

The balanced equation is:

Hg(NO₃)₂ (aq) + Na₂S (aq) → HgS (s) + 2 NaNO₃ (aq)