The slope of the line can be determined by finding the ratio of the change in y-coordinates to the change in x-coordinates between any two points on the line.
Using the points (1, 3) and (3, 9), the change in y-coordinates is 9 - 3 = 6 and the change in x-coordinates is 3 - 1 = 2.
Therefore, the slope of the line is 6/2 = 3.