I am looking at this answer that is causing me confusion but I am not able to comment on it to gain clarification. Within it A is a matrix declared as follows:
$$ A= \left[\begin{array}{ccc} 6 & -4 & 0 \\ 4 & -2 & 0 \\ -1 & 0 & 3\end{array}\right]$$
The person who answers the question proceeds to claim that $$ A - 2I = \begin{bmatrix} 4 & -4 & 0 \\ 4 & -4 & 0 \\ -1 & 0 & 3\end{bmatrix} $$
Is it not supposed to be
$$ A - 2I = \begin{bmatrix} 4 & -4 & 0 \\ 4 & -4 & 0 \\ -1 & 0 & 1\end{bmatrix} $$
and if so why not?
You are correct. Because $$ I_3=\begin{pmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix}, $$ each diagonal component should have $2$ subtracted from it; the $(3,3)$ entry has been missed.