You can use any two points that are on the line. However they give us two convenient ones, so let's use them.

Example 2

Find the slope of this line:

Example 3

Find the slope of this line:

Since there is no change in the vertical direction, all horizontal lines have a slope of 0.

Example 4

Find the slope of this line:

Vertical lines have no change in the x values, making the denominator zero. You can't divide by zero*, ever, so vertical lines have no slope. This is called an undefined slope.

* Go on, try it. Plug 4 ÷ 0 in your calculator…. what do you get?