$\begin{bmatrix} {\color{Magenta} e1} & {\color{Blue} e2} & 0 &0 \\ {\color{Red} e3} & {\color{Magenta} e4} & {\color{Blue} e5} &0 \\ 0 & {\color{Red} e6} &{\color{Magenta} e7} & {\color{Blue} e8}\\ 0 & 0 & {\color{Red} e9} &{\color{Magenta} e10} \end{bmatrix}$
Tridiagonal matrix of order 4x4 looks like this. Tri-diagonal means there are 3 diagonals including the main diagnonal. See that the elements {e3,e6,e9} fall in the same diagonal line(lower diagonal) and I have colored them with red. Similarly {e1,e4,e7,e10} are colored in magenta to indicate they belong to the same diagonal line(main diagonal) and {e2,e5,e8} are colored with blue because they also belong to same diagonal(upper diagonal).
Now question says that the elements in this matrix are stored starting with the elements of lower diagonal and we store them diagonal wise.
That is first we cover all the elements of lower diagonal then main diagonal and then upper. The array if indexed from 0 will look like :
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
e3 |
e6 |
e9 |
e1 |
e4 |
e7 |
e10 |
e2 |
e5 |
e8 |
Now from the matrix, element at (4,3) is e9. It is indexed at 2.