$A=\left[\begin{array}{ll}1 & 2 \\ 4 & 2\end{array}\right]$
$2 \mathrm{~A}=\left[\begin{array}{ll}2 & 4 \\ 8 & 4\end{array}\right]$
L.H.S. $=|2 A|=\left|\begin{array}{ll}2 & 4 \\ 8 & 4\end{array}\right|=8-32=-24$
R.H.S. $=4|\mathrm{~A}|=4\left|\begin{array}{ll}1 & 2 \\ 4 & 2\end{array}\right|=4(2-8)=-24$
LHS $=$ RHS