Question 464397
<pre>
Lines that go uphill to the right have slopes which are positive numbers.
Like these:

{{{drawing(200,200,-5,5,-5,5,graph(200,200,-5,5,-5,5),
line(-6,-5,7,2),line(-6,1,7,2), line(-6,2,7,5) )}}}

Lines that go downhill to the right have slopes which are negative numbers.
Like these:

{{{drawing(200,200,-5,5,-5,5,graph(200,200,-5,5,-5,5),
line(-6,5,7,-2),line(-6,-1,7,-2), line(-6,-2,7,-5) )}}}

Lines that are horizontal have slope 0  (zero), like these:

{{{drawing(200,200,-5,5,-5,5,graph(200,200,-5,5,-5,5),
line(-6,3,7,3),line(-6,-2,7,-2), line(-6,-4,7,-4) )}}}

Lines that are vertical have NO slope, like these:

{{{drawing(200,200,-5,5,-5,5,graph(200,200,-5,5,-5,5),
line(3,-6,3,7),line(-2,-6,-2,6), line(-4,7,-4,-8) )}}}

Vertical lines are the only lines with NO slope.
All other lines have slopes.

y = 8 is the equation of a horizontal line with slope = 0 
and y-intercept (0,8) and it has no x-intercept.  Like this:

{{{drawing(300,300,-10,10,-8,12,graph(300,300,-10,10,-8,12),
line(-11,8,11,8),locate(0,8,"(0,8)") )}}}

x = 8 is the equation of a vertical line with NO slope 
and x-intercept (8,0) and it has no y-intercept.  Like this:

{{{drawing(300,300,-8,12,-10,10,graph(300,300,-8,12,-10,10),
line(8,-11,8,11),locate(8,1.5,"(8,0)") )}}}

Edwin</pre>