Answer
$-21$
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 \[
\left[ {\begin{array}{cc}
6 & -3 \\
-5 & -1 \\
\end{array} } \right]
\] the determinant equals $6(-1)-(-5)(-3)=-21$