Question

http://prntscr.com/9o8wur

Answer 1

@UnkleRhaukus

Answer 2

\[x_{ij}\]
i: denote the row number
j: denote the column number
\[x_{42}\]
means the value in 4th row and 2nd column

Answer 3

which will be (7) in your question :)

Answer 4

Thank u

Answer 5

\[b=\begin{bmatrix}b_{11}&b_{12}\\b_{21}&b_{22}\\b_{31}&b_{32}\\b_{41}&b_{42}\end{bmatrix}\]