How to find the determinant of a matrix?

For the firs part of our class today we learnt how to find the determinant of special size matrices.

1. For 2X2 square matrix, [ a a1 

                                          a2 a3] : det (A) = aa3-a1a2

2. For 3X3 matrix: 1. Write out the first two columns on the right side of the matrix

                             2. Subtract the left three diagonals from the left three diagonals

3. Triangular matrix: the upper one has zeroes below the main entry, and the lower has zeroes above

                                multiply each number in the main entry of the matrix 

See the example problem