If you are taking point (a,b) and reflecting it across the x-axis (the horizontal axis), your x value is going to stay the same because you want the point on the same vertical line as (a,b). The y-coordinate is going to be opposite because you want a reflection and the opposite of b will this give you the same distance from the x-axis as b.
So the transformation is this: (a,b) -> (a,-b).
All this means is leave x the same and take the opposite of y.
