The line of reflectionis the line that passes in between the pre-image and the the mirror image (mirror reflection).
From the reflection shown in the given diagram, we can see that the reflection is a vertical reflection. which means that the line of reflection will be parallel to the y-axis.
To determine where the line of refletion passes through let's take two mirror points on each of the mirror images and find the mid point.
The mirror points are traced down as shown in the attached image.
On A' the mirror point is on x = 0
On A, the mirror point is on x = 2
Then we can find the mid point to determine the line of reflection.
Therefore the line of reflection is at;