Answer
$K[ V(H_{2}O)_{2} Br_{4}]$
Work Step by Step
The formula is written according to the rules of writing the formula for complex compounds.
1. Potassium is the cation whereas diaquatetrabromovanadate(III) is the anion. Cations are written first, then are anions.
2. Diaquatetrabromo is the ligand, and vanadate(III) is the metal.
3. In diaquatetrabromo, diaqua means two water molecules $(H_{2}O)_{2}$, and tetrabromo means $Br_{4}$
4. Vanadate (III) means, Vanadium with +3 oxidation state. Thus,
$K[ V(H_{2}O)_{2} Br_{4}]$
5. In order to check if the charges are balanced on the cation and anion, We take out the charge on the anion first. Water has no charge. Therefore:
$V= +3$ and $Br_{4} = 4 \times -1$
$+3 -4$
$-1$
Thus, Potassium has a +1 charge and both the anion and cations are balanced. The formula of the complex is:
$K[ V(H_{2}O)_{2} Br_{4}]$
