Access the Catalog in the TINspire Calculator application. Press [MENU]→Matrix & Vector to access the Matrix commands. Many of the items contained in the Matrix & Vector menu work with a matrix that you must first define.
Pressing [MENU]→Matrix & Vector→Determinant to pastes the Det command to the entry line. The easiest way to define a matrix as the argument for the Det command is via the Expression templates.
Follow these steps to accomplish this task:

Open the Expression templates.

Highlight the Variable Size Matrix template (see the first screen).

Press [ENTER] to open the Create a Matrix dialog box, as shown in the second screen. Use the
keys to set the number of rows and columns.

Press [ENTER] to paste a blank matrix with the specified dimensions to the argument of the Det command.

Enter each element of the matrix, pressing [TAB] each time to move to the next field.

Press [ENTER] to execute the command.
In the last screen two matrices are multiplied and the Last Answer feature is used with the Transpose command to transpose the rows and columns of my answer.