Answer
Reflection across the $y$-axis
Work Step by Step
The matrix of the initial triangle's coordinates is:
$B=\begin{bmatrix}0&2&2\\0&0&-4\end{bmatrix}$
Consider matrix $A$:
$A=\begin{bmatrix}-1&0\\0&1\end{bmatrix}$
Compute $AB$:
$AB=\begin{bmatrix}-1&0\\0&1\end{bmatrix}\begin{bmatrix}0&2&2\\0&0&-4\end{bmatrix}$
$=\begin{bmatrix}0+0&-2+0&-2+0\\0+0&0+0&0-4\end{bmatrix}$
$=\begin{bmatrix}0&-2&-2\\0&0&-4\end{bmatrix}$
We notice that the new triangle has the same $y$-coordinates, but $x$-coordinates of opposite sign, which means by multiplying by matrix $A$ we get a triangle reflected across the $y$-axis.
