Find any two points on a line. Points are labeled with "x" and "y" coordinates. The x coordinate indicates the position of the point in terms of the horizontal x axis, and the y coordinate indicates the position of the point vertically on the y axis.
Count the number of markings between the two points vertically. For instance, consider two points with the coordinates (5, 4) and (10, 6). The x coordinates are the first numbers in each set, and the y coordinates are the second numbers in each set. Subtract the first y from the second y to get the distance between the two y coordinates. In this case you would subtract 4 from 6 to get 2.
Count the number of markings between the two points horizontally by subtracting the first x coordinate from the second x coordinate. In this case, you would subtract 5 from 10 to get 5.
Write your answers from Steps Two and Three as a ratio, with the difference in y coordinates stated first. In this example, you would write that the ratio of the slope was "2 to 5," meaning that the line rises two points each time it runs five points.