Reflection over a line
We are given the drawing of a triangle ABC, and it's required to reflect it across the line x = 1. We have completed the figure with all the information we need to perform the task. Note the line x = 1 passes through point A, this point maps to itself.
Reflecting a point across a given line means to project is as if the line was a mirror and our point maps to a point that is at the same distance to the mirror.
For example, point B is 3 units away from the mirror, thus we map it to a point located 3 units 'behind the mirror', that is at x = -2.
The original triangle, the mapped triangle, and the line of reflection are shown in the figure below:
Summarizing:
Point A(1, 2) maps to itself A'(1, 2)
Point B(4, 2) maps to B'(-2, 2)
Point C(2,4) maps to C'(0, 4)