Answer
$-250$
Work Step by Step
Since we have $$\left[\begin{array}{rrr} {5} & {15} \\ {10} & {-20}\end{array}\right]=5\left[\begin{array}{rrr} {1} & {3} \\ {2} & {-4}\end{array}\right]$$
then, we get
$$\left|\begin{array}{rrr} {5} & {15} \\ {10} & {-20}\end{array}\right|=5^2\left|\begin{array}{rrr} {1} & {3} \\ {2} & {-4}\end{array}\right|=25(-4-6)=-250.$$
