Please help with matrices question(30) in the below packet.

*Appreciate it if you can show your work

If you are unable to view the question below, here is the link https://massey.limfinity.com/207/hillcipher.pdf

Thanks

Conclusion Will Gaussian elimination always work? The answer is no - some matrices do not have inverses, and the algorithm will get stuck. Take a course in linear algebra to learn the details. It turns out that the Hill Cipher can be broken. If an enemy knows the plaintext and corresponding ciphertext of a single message, she can reverse engineer to discover the key, with which to decrypt future messages. Fortunately for modern electronic communication, mathematicians have developed more sophisticated and secure methods of encryption. They all build upon the basic ideas we have learned in discrete math. 30. (Bonus) The plaintext "BONUS PROBLEM" gets encrypted to "QF43406. IFJJXTS", and the Hill Cipher key is nine characters long. Decrypt the message "V. 8Y_Q063163. 15"