Hi there, I have a homework problem where I am supposed to decrypt a Hill 2-cipher. The word I am supposed to decrypt is

XP PX RL VH

it is ciphered using digraphs on 26 letters (ignoring cases). (a=0, b=1,...,z=25)

The encryption matrix is a 2x2 matrix E.

E=(2 1)

(3 5)

The problem says that to find the decryption key, i take the inverse of matrix E in the sense of modulo 26.

i know that the message i am supposed to get in the end is

SN AP SH OT

but i am completely lost on how to take the modular inverse of a matrix.