Question 310670
<pre><font size = 4 color = "indigo"><b>
{{{drawing(400,400,-5,5,-5,5,

graph(400,400,-5,5,-5,5),
locate(1,4,"(1,4)"),
locate(1,1,"(1,1)"),
green(line(1,-6,1,6)), line(1-.1,4,1+.1,4),  line(1-.1,1,1+.1,1) )}}}

The line is vertical and therefore has no slope.  Vertical lines are 
the only kind of lines that have no slope (or perhaps your teacher
says "Their slope is undefined".)

Edwin</pre>