I think you're missing something from your initial equation. The equation you wrote doesn't match what Yusuke asked for.2Hg is incorrect, Hg already gives off 2 electrons. 2Hg would mean that Cl becomes 4Cl, unless that's what your teacher wants. Not only that, but Hg has a positive charge, not a negative one. If it did, it wouldn't be able to pair up with either nitrate or chloride.
He says that there are originally 2 Hg's on both sides.Yusuke said:Write net ionic equation to show the reaction of aqueous Hg2(NO3)2 with aqueous sodium chloride to form solid Hg2Cl2 and aqueous sodium nitrate.
So what he asked for is:
Hg[SUB]2[/SUB](NO[SUB]3[/SUB])[SUB]2[/SUB] [SUB](aq)[/SUB] + 2 NaCl [SUB](aq)[/SUB] → Hg[SUB]2[/SUB]Cl[SUB]2[/SUB] [SUB](s)[/SUB] + 2 NaNO[SUB]3[/SUB] [SUB](aq)[/SUB]