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