Answer
Hg$_{2}$CO$_{3}$
Work Step by Step
Hg$_{2} ^{2+}$ has a 2+ charge, and CO$_{3}^{2-}$ has a 2- charge. You can see that the two balance each other out to create a neutral compound. Therefore you need one Hg$_{2} ^{2+}$ and one CO$_{3}^{2-}$ to get a compound with a net 0 charge, and the chemical formula must be Hg$_{2}$CO$_{3}$.
