Answer
$-29$
Work Step by Step
The general formula to calculate the determinate of a $2 \times 2$ matrix which has 2 rows and 2 coulmns is
$D=\begin{vmatrix}a&b\\c&d\end{vmatrix}=ad-bc$
Now,
$D=\begin{vmatrix}-4&1\\5&6\end{vmatrix}=-24-5=-29$