It is a very simple vignere challenge:
You know the flag starts with RTL, vignere is similar to xor: 
MSG x KEY = CRYPT
CRYPT X KEY = MSG
CRYPT X MSG = KEY
So you try decrypting `NBY{9x175777156k5608n3x889n5nx9215n2}` using vignere with the key `RTL`, and you get `WIN{9g175777156r5608c3g889u5cg9215u2}`, you could remove the flag part and only try decrypting `NBY` (crypt) with `RTL` (msg), and you'd get `WIN` (key), so you try decrypting `NBY{9x175777156k5608n3x889n5nx9215n2}` using vignere with the key `WIN` and you get the flag