When you reflect a point across the y-axis, the y-coordinate remains the same, but the x-coordinate is transformed into its opposite (its sign is changed). Notice that B is 5 horizontal units to the right of the y-axis, and B’ is 5 horizontal units to the left of the y-axis. the y-axis is the point (-x,y).

What is the rule for the reflection?

The rule of reflection is very simple and logical. It says that when a point (x,y) is reflected along the x-axis, the y- coordinate becomes negative and when the same point is reflected along the y- axis, the x coordinate becomes negative.