The 2×2 determinant ad − bc measures signed area of the parallelogram spanned by the columns (and whether the matrix is invertible). For [[a,b],[c,d]] it is simply the product of the main diagonal minus the product of the off-diagonal.

Steps

1. For [[a, b], [c, d]], form ad − bc.
2. Watch the minus sign between the two products.
3. Enter the resulting integer.