I'm getting a bit mixed up with what these notes are referring to for $A$.
Here's an example :
This is expressed in tableau as follows
Which, I think, should be found from the following :
But in the top middle cell there is $A_B^{-1}A_N$, and I'm not sure what would be $A_N$ for the example problem.
Assuming that, for the example, the matrix is
$$ A = \pmatrix{1 & 2 \\ 1 & -1} $$
Which is what I feel that it should be, but that table with $A_B^{-1}A_N$ is confusing me a bit