RSA: How to compute plaintexts knowing ciphertexts, public key and relation between plaintexts
Since this is an exercise, I'll provide the links of the articles as a hint;

The linearity of the messages  is first studied when $e=3$


Low-Exponent RSA with Related Messages by Don Coppersmith, Matthew Franklin, Jacques Patarin, Michael Reitert, EUROCRYPT 1996: Advances in Cryptology — EUROCRYPT ’96 pp 1-9


A more general case, where $e$ is not limited to 3 and the relation is linear is studied in


A New Related Message Attack on RSA Oded Yacobi and Yacov Yacobi, PKC 2005: Public Key Cryptography - PKC 2005 pp 1-8. 


It is quite clear that these attacks are related to textbook RSA and it must not be used in practice. 

For encryption, RSA has used either PKCS#1.5 padding or OAEP. Actually, we don't use RSA for encryption. We prefer it in digital signatures and that requires PSS padding or for Key Encapsulation, RSA-KEM.