Answer
Coordination number: 6
Oxidation number: +3
Work Step by Step
The ligand ($CN_{6}$) that surround the central metal atom $Co$ are 6. Thus, the coordination number is 6.
Let us suppose the oxidation number of $Co$ is X. To find the oxidation number, we sum the known oxidation numbers and equal it to the charge on the complex.
Also,
$K_{3}= 3\times (+1)$
$CN_{6}= 6 \times (-1)$
The overall charge is zero on the complex, thus:
$3\times (+1) +X + 6 \times (-1) =0$
$+3 +X -6 = 0$
$X= +6 -3$
$X= +3$
The oxidation number of $Co$ is +3
