When you are given two points of a line, you can find the slope of the line.
Let's say that the two points we are given is

, then the slope is:

Another thing to keep in mind:
When you have 0 in the numerator, and a number in the denominator, that means that the slope is 0.
When you have 0 in the denominator, and a number in the numerator, that means that there is no slope, or that the slope is undefined.