Question 154168
Why is x = 4 a vertical line?
It's a line parallel to the y-axis, which is usually drawn as vertical.
It's parallel to the y-axis because the x value is the distance from they y-axis (just as the y value is the distance for the x-axis), so it's a constant distance from the y axis, which is the definition of parallel.
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To put it another way, no matter what the value of y, x=4.  Since it's a straight line, if you find 2 points and draw it through them, that's the graph.  For x=4, 2 points (out of an infinite number) are (4,0) and (4, 10).  If you connect these 2 points, the line will be vertical.