This reaction has one molecule of oxygen on the reactant side and at the product side, there are two molecules of oxygen. So let us first make the oxygen molecules two on the reactant side.

2HgO → Hg + O₂

Now there are two molecules of mercury on the reactant side and only one molecule on the product side. So the total balanced reaction is as follows:

2HgO → 2Hg + O₂

It is thermal decomposition reaction as lead oxide decomposes on being heated to give lead metal.