So, let's think about it. The y-axis runs vertically, right? What you are basically doing when reflecting is picking up your image, flipping it over, and placing it equidistant on the other side of your reflection point or line or whatever.
Reflecting across the y-axis, the image isn't moving anywhere vertically, so the y-values stay the same. The x-values all switch signs.
Let's take one point, like (6,3). Reflecting that across the y-axis gives us (-6,3).
