y = 3 is the line of reflection
Under a reflection in a line, corresponding points of the original and it's image should be equidistant from the line of reflection
y = 3 is the horizontal line passing through all the points with a y-coordinate of 3
A is 1 unit above and A' is 1 unit below the line
B is 3 units above and B' is 3 units below
C is 1 unit below and C' is 1 unit above
D is 1 unit below and D' is 1 unit above
The vertices of the figure and the image are all equidistant from y = 3
