19. Consider the two matrices A, Q, where

$$

A = \begin{bmatrix} 1 & 2 \\ 3 & -1 \end{bmatrix}; \qquad Q = \begin{bmatrix} 2 & 1 \\ 4 & 1 \end{bmatrix}.

$$

Show that |A|=|Q^-1AQ| for these matrices and thus demonstrate Theorem 3.3.1