I have to create a procedure that decodes an encoded message using a 2x2 matrix and a row vector.  (Alphanumeric is a 95 character string)

So far I have A:=Matrix(2, 2, [[17,22], [4, 53]]); B:=Vector[row]([19,82]); as my two matricies, and have writtten this as my proc:

Can someone explain how to do this?

Prove that if we describe the circle of center (a; b) and radius r using
the parameters (a; b; k), with k = a2+b2-r2, rather than the more natural parameters
(a; b; r), then the error function H(a; b; k) = E(a, b,rad(a^2 + b^2 -k) is quadratic in a; b
and k. What does this imply about the number of critical points?