The oxidation state of iron in Na₄[Fe(CN)₅(NOS)] is :

The compound given is: Na_4​[Fe(CN)_5​(NOS)], 

Let, oxidation number of Fe=x

Oxidation number of CN=−1

Oxidation number of NOS=−1

Therefore, x+(−1)×5+1(−1)=−4

                                    x−5−1=−4

                                     x=+2