Reflection over the y-axis is a type of geometric transformation that moves every point on an object to a new position. Imagine the y-axis as a mirror. Every point will have an image on the opposite side of the y-axis at the same distance from the y-axis.
If a point P(x,y) is reflected over the y-axis, its image's coordinates, P′(x′,y′), will follow the rule:
x′=−x
y′=y
Thus, the image of point P(x,y) is P′(−x,y).
Note that the value of the y-coordinate does not change, while the value of the x-coordinate becomes its opposite (negative if positive, positive if negative).
Suppose we have a line with the equation y=x+2. To find the equation of its image after reflection over the y-axis, we substitute x with −x (because x′=−x) and y with y (because y′=y) into the original equation.