Question 249138
The points (4,-5)(4,-1)(4,7) have what in common? They have the value of 4 as the x coordinate. So we'll get a vertical line. 



If you plot those points you get




{{{ drawing(500, 500, -10, 10, -10, 10,
 graph( 500, 500, -10, 10, -10, 10,0),
 circle(4,-5,0.05),circle(4,-5,0.08),circle(4,-5,0.10),circle(4,-5,0.12),
 circle(4,-1,0.05),circle(4,-1,0.08),circle(4,-1,0.10),circle(4,-1,0.12),
 circle(4,7,0.05),circle(4,7,0.08),circle(4,7,0.10),circle(4,7,0.12)

)}}}



and draw a straight line through them, you get



{{{ drawing(500, 500, -10, 10, -10, 10,
 graph( 500, 500, -10, 10, -10, 10,0),
 blue(line(4,-20,4,20)),
 circle(4,-5,0.05),circle(4,-5,0.08),circle(4,-5,0.10),circle(4,-5,0.12),
 circle(4,-1,0.05),circle(4,-1,0.08),circle(4,-1,0.10),circle(4,-1,0.12),
 circle(4,7,0.05),circle(4,7,0.08),circle(4,7,0.10),circle(4,7,0.12)

)}}}



So the graph of {{{x=4}}} is the vertical blue line. Basically, the value of 'x' is fixed at 4 (ie it cannot be any other value), but y can be any value which gives us this vertical line.