Answer
$BZ=\left[\begin{array}{l}
-x\\
y
\end{array}\right]$
$(x,y)\mapsto(-x,y)$ is a reflection about the y-axis.
Work Step by Step
$CZ=\left[\begin{array}{ll}
-1 & 0\\
0 & 1
\end{array}\right]\left[\begin{array}{l}
x\\
y
\end{array}\right]=\left[\begin{array}{l}
-1(x)+0y\\
0x+1(y)
\end{array}\right]=\left[\begin{array}{l}
-x\\
y
\end{array}\right]$
If the point (x,y) is transformed into ($-x,y$),
it has been reflected about the y-axis.
Reflecting all vertices about the y-axis results in reflecting the graphic about the y-axis.
