Answer
$16$
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 $D=xb-ay=8$ is given.
Our determinant is: $2xb-2ya=2(xb-ay)=2(8)=16$
