The oxidation number of Cobalt in ${\mathrm{K}}_3\left[\mathrm{Co}(\mathrm{CN}{)}_6\right]$ is +x,  value of x is

Let x be the oxidation state of Co in [Co(CN)₆​]³⁻.
Since the overall charge on the complex is −3, the sum of oxidation states of all elements in it should be equal to −3.
Therefore, x+6(−1)=−3
or, x = +3.