Answer
$\left[\begin{array}{lll}
700 & 1300 & 2000\\
400 & 300 & 500
\end{array}\right]$
represents sales in January,
first row:SF, second row: LA
Inventory remaining = $\left[\begin{array}{lll}
300 & 700 & 3000\\
600 & 4700 & 1500
\end{array}\right]$
Work Step by Step
Let $A=\left[\begin{array}{lll}
1000 & 2000 & 5000\\
1000 & 5000 & 2000
\end{array}\right]$ represent current inventory,
$B=\left[\begin{array}{lll}
700 & 1300 & 2000\\
400 & 300 & 500
\end{array}\right]$
represent sales in January, first row:SF, second row: LA
Inventory remaining = $A-B$
$=\left[\begin{array}{lll}
1000 & 2000 & 5000\\
1000 & 5000 & 2000
\end{array}\right]-\left[\begin{array}{lll}
700 & 1300 & 2000\\
400 & 300 & 500
\end{array}\right]$
$=\left[\begin{array}{lll}
1000-700 & 2000-1300 & 5000-2000\\
1000-400 & 5000-300 & 2000-500
\end{array}\right]$
$=\left[\begin{array}{lll}
300 & 700 & 3000\\
600 & 4700 & 1500
\end{array}\right]$
