I have the same question for a vertical line.
Unless you repeat the question, it cannot be answered from this post.
I got the question from your previous post. A vertical line has no slope.
http://www.purplemath.com/modules/slope.htm
http://wiki.answers.com/Q/What_is_the_slope_of_a_vertical_line
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
I hope this helps.