The line y = 3 is shown as a line moving horizontally across the value 3 on the y-axis. It would move across the coordinate (0, 3).
To reflect points across a line, you basically are plotting opposites. Look at it as if it were a mirror; if you plot (7, -4) on a graph with a line of y = 3, you will find that it is 7 points below y = 3. To reflect (7, -4) across y = 3, you now know you must put it the same amount of points above y = 3, so in this case it will be 7 points above y = 3.
I attached an image to represent the solution. Hope it helps!
