The oxidation number of Fe in  ${\left[Fe{\left(CN\right)}_6\right]}^{4-}$; $Cr$  in $\left[Cr{\left(N{H}_3\right)}_3{\left(N{O}_2\right)}_3\right]$ and Ni in $Ni{\left(CO\right)}_4$ are respectivey

${\left[Fe{\left(CN\right)}_6\right]}^{4-}$

$x+6(-1)=-4$

$x-6=-4;x=6-4=2$

oxidation number of Fe = +2

$\left[Cr{\left(N{H}_3\right)}_3{\left(N{O}_2\right)}_3\right]$

$x+3\left(0\right)+3(-1)=0$

$x-3=0;x=+3$ oxidation number of Ni = 0