The line x = 2 is a vertical line. Since this vertical line is to be the perpendicular bisector of the line joining A and B, the line joining A and B must be a horizontal line. This means that the y coordinate of point B must be the same as that of point A - that is 1. The remaining issue is to find the x coordinate of point B. Since -4 (the x coordinate of point A) is to the left of the line x =2, the x coordinate of point B must be to the right of the line x = 2. We want the point (2, 1) to be the mid point of the line joining A and B. The coordinates of B which do that are (8, 1). The value 2 is the average of 8 and -4, and points A, B and the intersection of the two lines all have the same y value - that is 1.