Explanation
Locate the lowest point on the upper triangle. It is at (-2,-1)
It reflects over the mirror line to land on the highest point on the lower triangle which is the point (-2,-3). Note the swap from highest to lowest.
The midpoint of these points is (-2,-2). We add up the y coordinates and then divide in half. The x coordinate remains the same. This only applies when the endpoints are vertically aligned.
Follow similar logic for the other pairs of mirrored points. What do you notice? You should notice that all midpoints are on the same horizontal line through -2 on the y axis. Therefore the mirror line equation is y = -2
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Rephrased another way:
Start at (-2,-1) and notice we move 2 units down to reach its reflected point (-2,-3). Half of this vertical distance is where the mirror line will be. Start at y = -1 and move down 1 unit will have you arrive at y = -2. The same will apply for the other pairs of points as well.
This trick only works because the paired mirror points are vertically aligned.
See the diagram below.
