What is the value of the element at row 3 column 1?

4

The numbers of rows and columns of a matrix are
called its **dimensions**.
Here is a matrix with three rows and two columns:

Sometimes the dimensions are written off
to the side of the matrix, as in the above matrix.
But this is just a little reminder and not
actually part of the matrix.
Here is a matrix with __different__ dimensions.
It has two rows and three columns.
This is a different "data type" than the previous
matrix.