sanjib wrote:Is the line parallel to y axis?
1.the equation of the line is x.

the vertical line's slope is undefined because the denominator is 0.So how come the vertical line equation become x=4?

Pls help.Am I missing something?

The equation of y axis is x=0

Any point that lies on it has to have the x coordinate as 0.
You are right that the slope becomes undefined as the denominator is 0.
Take any two points to calculate the slope of a vertical line.

Lets take x=4 for example.

points be (4,1) and (4,4)

slope = (4-1)/(4-4) = 3/0 => which becomes undefined. So, it is perfectly possible for a line to have an undefined slope.

Little difficult to explain without sketches of a line, but i hope it helps a bit.