Find the slope of this line:
You can use any two points that are on the line. However they give us two convenient ones, so let's use them.
Since there is no change in the vertical direction, all horizontal lines have a slope of 0.
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?
Make it rain.