Find the slope of the line that passes through each set of points

You didn't provide any points, but you'd need only two per line.

If the points are (a,b) and (c,d), the slope would be (d-b)/(c-a).

Slope really means "how much does the y change when the x advances by 1?"

In above two points, the y-difference is d-b. However, the x-difference is not 1, but it is c-a. If we divide by c-a, we get the y difference as if x advanced by 1.