Answer
$-8$
Work Step by Step
We know that for a matrix
\[
\left[\begin{array}{rr}
a & b \\
c &d \\
\end{array} \right]
\]
the determinant, $D=ad-bc.$
Hence here $D_1=xb-ya=8.$
We are looking for $D_2=ya-xb=-(xb-ya)=-8$