Find the indicated submatrix? (Simple, I'm missing something obvious) So I'm presently looking through my professor's notes and I cannot figure out for the life of me the sub matrix. I've never seen a matrix where there is a horizontal line through it like this. > **Identify the indicated submatrix** $$ A = \left[\begin{array}{rrr|r} 0 & 1 & -7 & -5\\\ 7&-1&0&2\\\ \hline 2 & 5 & -2 & 0\end{array}\right]$$ Find $A_{12}$.

I think they intend: You can break $A$ into four submatrices of size $2\times3$, $2\times1,$ $1\times3$, and $1\times1$. Which one is $A_{12}$?