If you are reflecting across the line y = x, then the x value becomes the y-vale and the y-value becomes the x-value. So the answer would be (6, -1) (Just trade places with x and y.)
A horizontal line would mean that it has no slope, since the line is not decreasing or increasing. The horizontal line drawn through (0, -1) would pass through (-2, -1), since they share the same y coordinate.
