I have found that calculating the determinant of a 3x3 matrix with vector operations is easier to remember, easier to express and easier to code than the usual way it is learned.

Given a 3x3 matrix:

We construct the following three vectors, one for each row of the matrix:

The determinant of the matrix can be calculated with any of the following formulas:

If we take one vector for each column of the matrix, the same formulas apply. The expression is called scalar triple product.

An implementation example in pseudo-code:

float det(Matrix3x3 m)
{
	return dot(m.row(0), cross(m.row(1), m.row(2)));
}