Answer:
y = -1
Explanation:
Because the coordinate of U and U' is (5, 0) and (5, -2), the line that is right in the middle would be y = -1.
Or you can compare two of the coordinates to create the line of reflection.
For example: R (0,0), R'(0, -2)
U(5, 0), U'(5, -2)
The middle points are (0, -1) (5, -1).
In this case, y = -1.