Answer
$ [Co(NH_{3})_{4}CO_{3}]_{2} SO_{4}$
Work Step by Step
The formula is written according to the rules of writing the formula for complex compounds.
1. Tetraamminecarbonatocobalt(III) is a cation whereas sulfate is the anion. Cations are written first then anion.
2. Tetraamminecarbonato is the ligand and cobalt (III) is the metal. Thus, $[Co(NH_{3})_{4}CO_{3}]$
3. Cobalt (III) means the oxidation number of cobalt is +3. $NH_{3}$ is a molecule, it has no charge. Carbonate has a -2 charge. Thus, the charge on the complex is $+3-2 = +1$
4. The charge on Sulfate is -2. In order to balance the charges on the cation and anion, two atoms of the complex should be present. Therefore, the formula of the complex should be:
$ [Co(NH_{3})_{4}CO_{3}]_{2} SO_{4}$