Well, it's pretty simple. If your reflecting something over the y axis the X will change if it's negative or positive. So if it's -x and you flip it over it becomes a positive x. If you flip it over the x-axis the Y is the one that change to a negative or positive.
So if the shape flips over the y-axis, the X points will turn negative. for example, one of the points is (1,4) it will turn to (-1,4)
