Explanation:
The rule [tex](x,y) \to (-x,y)[/tex] has us flip the sign of the x coordinate from positive to negative (or vice versa). This applies a reflection over the y axis. For example, the point (5,6) moves to (-5,6).
Currently this shape is in quadrant 1. The quadrants start in the top right corner, and you move counterclockwise going to quadrants 2,3, and 4. Quadrant 2 is to the left of quadrant 1, and this is where all of the points end up after applying a reflection over the y axis.