What is $\begin{vmatrix}2 & 5 & -1 \\ 0 & 3 & 4 \\ 0 & 0 & -2\end{vmatrix}$?

Explanation

1. 01

   Every element below the leading diagonal is 0. Expanding down column 1 leaves $2\begin{vmatrix} 3 & 4 \\ 0 & -2 \end{vmatrix}$.

2. 02

   $= 2(-6 - 0) = -12$: the product of the diagonal elements $2 \times 3 \times (-2)$.