What are the products of the reaction of NH4Br (aq) + Hg2(NO3)2 (aq)?
According to this table, mercury (I) bromide ($\ce{Hg_2Br_2}$) is somewhat insoluble (among other mercury (I) salts), so it gets precipitated as $\ce{Hg_2Br_2(s)}$ instead of $\ce{Hg_2Br_2(aq)}$, making the overall equation this:

$$\ce{2NH4Br(aq) + Hg2(NO3)2(aq) -&gt; 2NH4NO3(aq) + Hg2Br2(s)}$$



Mercury (I) ions are insoluble because every orbital is full, making it unable to form ligands with water.