This is a question on a quiz, so I'm not looking for the answer, just a pointer in the right direction.

I have the following matrix:

5 2

-1 3

I computed the characteristic polynomial and got the following eigenvalues:

(i-4) and (i+4)

Unfortunately, the matrix that I got by multiplying (i-4) by identity(3) and subtracting the original matrix reduces to identity(3) according to my calculator. (i+4) does the same.

Uuh? What are you doing here? Do you mean ?? And even if you get this, what are you doing it for?? What are you trying to achieve? Tonio
Can anyone help, or do I need to provide more information?