Answer:
x = 4
Explanation:
A line perpendicular to the x-axis is a vertical line. A vertical line has the equation ...
... x = <some constant>
If you want the vertical line to go through a point with an x-coordinate of 4, then the consant must be 4.
... x = 4
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Comment on Equations of Lines
Often you will see the equation of a line written in slope-intercept form
... y = mx + b . . . . . . . slope = m; y-intercept = b
or in point-slope form
... y -k = m(x -h) . . . . . . slope = m; point = (h, k)
These work well enough when the slope is defined (that is, not "undefined"). A vertical line, one perpendicular to the x-axis (or parallel to the y-axis) has undefined slope, so cannot be written using an equation of the above form.
Another form of the equation of a line is "standard form"
... ax +by = c
where a, b, and c are integers that may be zero. When b=0, the line is a vertical line, equivalent to x = c/a. This form of the equation of a line can be used when the slope is undefined. (It can also be a useful form for other reasons, as well.)