The equation of a line that passes through (3,5) and (3,9) is x = 3. This can be found using the point-slope formula: y-y1 = m(x-x1).
It is perpendicular to the x-axis. The line intersects the x-axis at a right angle
Since the line is vertical, the slope is undefined. Slope is equal to rise/run, or increase in y/increase in x. Because there is no increase in x, the slope would be equal to 1/0, which is undefined, because we cannot divide by 0.
Hope this helps! :]
