Answer
$Cs [Cr(NH_{3})_{2}(CN)_{4}]$
Work Step by Step
The formula is written according to the rules of writing the formula for complex compounds.
1. Cesium is a cation whereas diamminetetracyanochromate(III) is the anion. Cations are written first, then are anions.
2. diamminetetracyano is the ligand whereas chromate(III) is the metal. Metal is written first then is the ligand.
3. In diamminetetracyano, diammine means two amine $(NH_{3})_{2}$, tetracyano means four cyanide $(CN)_{4}$
4. From point 2 and 3 we have: $[Cr(NH_{3})_{2}(CN)_{4}]$
5. Chromate(III) means chromate has an oxidation state of +3. Cesium has a +1 charge. To find the charge of the cation:
diammine charge is 0, tetracyano charge is $4\times (-1)$
$+3-4$
$-1$
Thus charges are balanced, The formula is:
$Cs [Cr(NH_{3})_{2}(CN)_{4}]$