y = -1 is the equation of a horizontal line that passes through every point with -1 as its y-coordinate. In other words, every point on the line takes the form (x, -1).
A line perpendicular to this one would be vertical, so that the x-coordinate is now fixed. If this line passes through (8, -4), then for every possible y-coordinate, the x-coordinate is fixed to be 8. So every point on the line takes the form (8, y), and the equation for this line is x = 8.
