If it's been awhile since you've studied geometry, please review this Background Material/Prior Knowledge.
Use the GeoGebra experiment below or linked here to experiment with reflections.
Use your work above to write a definition of a reflection.
Now experiment with pentagon ABCDE shown below or linked here.
What happens to points between the line segments' endpoints?
How does the length of a segment compare to that of its image?
In which direction are vertices A, B, C, D, and E oriented? How about their images after a reflection about line l?
Check your work here.
Suppose you are given point A and its reflection image A'. How might you construct the line of reflection? Use Desmos to do it. (That is, draw points A and A' and construct the line of reflection.)
If the reflection image of A over line l is B, what is the reflection image of B over line l? Why?