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Slope of Line 
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Let’s see what happens when we try to calculate the slope of a vertical line.
Choose any two 2 points on the line: (3,1) and (3, -2) 

Find slope using the slope formula: 

Step 1 Label (3, 1) and (3, -2)

Step 2 Plug the numbers into the formula

Finding slope graphically

Slope of a Horizontal Line

Slope of a Verticall Line

Slope of a Line

The slope of a horizontal line is zero. A horizontal line has a horizontal change but the vertical change is zero. The rise/run formula for slope always yields zero since rise = 0.
Let's look at an example:

The y intercept of a Horizontal Line is calculated using the 

y = mx + b ( The slope will be the where the line intercepts the y axis)
y = 0x + 2
y = 2

If the line crosses the y axis at -3 
y=0x -3

A line that is vertical does not  have a slope A vertical line has a "rise", but no "run". The rise/run formula for slope always has a zero denominator and is undefined.
Because a Horizontal line always has a rise of 0 it will always have a slope of 0
= Undefined Slope
= Undefined Slope
-7    -6    -5    -4   -3   -2    -1
Video Example of finding slope using the slope formula of line passing through (3.1) and (3 -2)