The line x = 2 is vertical. This means that ab is horizontal since they are perpendicular. So, the y coordinate of b must be 1.
If x = 2 bisects ab, then a and b are equal distances from x = 2.
a is 2 - (-4) = 6 units away from x = 2, so b must be 6 units away in the other direction. The x coordinate of b is 6 + 2 = 8.
The coordinate of b is (8, 1)
