Using the same encryption code C, the student’s friend answered with 

What was the return message? (See Exercise 43.) 

Data from Exercises 43

An avid math student sends an encrypted matrix message E to a friend who has the coding matrix C. The friend can decode E to get message M by multiplying E by C⁻¹. If

what is the message? (0 = space, 1 = A, 2 = B, 3 = C, etc.)

R= = 19 9 20 23 70 83 67 88 45 43 40 64 39 30 45 51