The answer is: the Y axis. :I
You reflect it, then you shift the shape 1 unit to the right. For example, Point U is at coordinates, -2, 2. Reflected across the Y axis, it's 2, 2. Then to get to the point that U' is at, you shift your reflected U to the right one unit. That gets you to coordinates 3, 2. Same thing goes for the other Points. Cx Hope this helped!!
