Application of linearity

How can Bob decrypt the message?

Assuming that Matrix A is what Alice wants to say, and matix B is a 3-by-3 matrix to encrypt Alice's message, and matrix C is the encoded messange.

It can be expressed A*B=C.

In order to find A, Bob needs to find the inverse matrix of B.

A*B*B`= C*B`, A*I=C*B'

After that, he needs to calculate C*B`. By finding its corresponding order in the alphabet, He can figure our what she wants to say.