The line y = -2 is a horizontal line through -2 on the y axis.
Anything perpendicular to this horizontal line will be a vertical line. All vertical lines are of the form x = k where k is some fixed value.
In this case, the answer is x = 4 because want the vertical line to go through (4,-2). In other words, we want all points on the vertical line to have x coordinate of 4.
Side Note: the slope of y = -2 is 0 while the slope of x = 4 is undefined.
