According to the dimensions of the matrix, you have just to insert the numbers in the proper cells and after press the corresponding calculation button:
For the 2x2 fill please the green boxes and after press the button "2x2"
For the 3x3 fill please the blue boxes and after press the button "3x3"
For the 4x4 fill please the violet boxes and after press the button "4x4"
The cells without numbers or empty are considered equal to Zero.
If the determinant is equal to Zero, the matrix is not invetible and therefore it is indicated with the "!" in the cells of the inverse matrix.