I am using a casio fx9750ga plus. I cannot get

Hello,
I assume yoy know how to define your matrix, but I will repeat it here for others who might not know. You can skip to Calculation
1.Turn Calculator ON. If there are no icons, press [MENU].

Data Entry
2.Use arrows to highlight [MAT] icon. Press [ENTER]
3.Highlight the first line where it says Mat A. Use the right arrow to enter the dimensions of Matrix A. Enter 3 and press [EXE]. The cursor moves to the second dimension. Press 3 and press [ENTER].
4. The matrix entry screen appears.
5. Enter first matrix coefficient a_11 and press [ENTER]
6. Enter a_12 and [ENTER]
.....
Key in last coefficient a_33 and press [ENTER]

Calculation
Press [MENU] and [RUN]
Press [OPTN][F2:MAT][F3:Det] Command echoes on screen as det
Press [F1:Mat] echoes on screen as Mat; screen shows det Mat
Press [ALPHA] A The screen displays det Mat A
Press [ENTER] to calculate the determinant.

If the matrix is not square, you cannot calculate its determinant.

Hope it helps..