I'm not familiar with this notation, but believe it means that you are to reflect point P over the line y=1. If you were reflecting P over the x-axis (line y=0), the result would be (-2,-7). However, you are to reflect P over the line y=1. The y-coordinate of (-2,7) is 7, which is (7-1), or 6, units above the line y=1. So, going in the other direction (reflecting P in y=1), you count 6 units down from y=1. Note how 1 + 6 = 7 units. New coordinates are (-2,-6).
