This is the 2911th most frequent Polish word.
mojego
"Of my" or "my" (possessive, masculine/neuter/genitive).
The word 'mojego' is used in possessive form to indicate 'of my' and is in the genitive case matching the noun 'brata' (brother) which is also in genitive.
To jest obraz mojego brata.
This is a picture of my brother.
Here 'mojego' indicates possession, 'my', and aligns with the noun 'przyjaciela' (friend) which is in the genitive case due to the verb 'widziałem' requiring this case in negative forms.
Nie widziałem mojego przyjaciela.
I have not seen my friend.
The possessive 'mojego' is used in conjunction with the noun 'samochodu' (car) in genitive case as it follows the genitive rule after 'kolor'.