Answer
$4$
Work Step by Step
For any matrix \[
\left[ {\begin{array}{cc}
a & b \\
c & d \\
\end{array} } \right]
\] the determinant equals $ad-bc$. For the matrix \[
\left[ {\begin{array}{cc}
2 & -4 \\
-1 & 0 \\
\end{array} } \right]
\] the determinant equals $2(0)-(-1)(-4)=4.$