Reflectory: flip or mirror image of a shape across a line of reflection.
The flipped image is reflected perpendicularly across the line of reflection
Lines connecting mirror points will be perpendicular to the line of reflection
mirror points are equidistant from the line of reflection
How to algebraically determine mirror points:
Use the equation of the line of reflection and vertices of the shape to solve for the perpendicular lines.
Use the midpoint formula and this equation from 1) in order to find the coordinates knowing that the line of reflection is the midpoint of the mirrored point.