Answer: (x,y) -> (x,-y)
Explanation:
If you take any point, for example (-1,1) from the blue shape, you have to multiply the y value by -1 to get to (-1,-1) for the red shape. You can test it out with any point on the two figures.
Also, by looking at it, you can see that it is a reflection over the x axis and a reflection over the x axis always just multiplies the y value by -1.