All vertical lines have an undetermined slope (since the formula which represents the slope has a denominator of 0). Therefore the general formula of a vertical line is the following x = c, with c representing the x-coordinate of the point that is passed by the line. Therefore, in your situation we get the line x = 3. When we plot this line, we can see it is a vertical line with x-coordinate 3 and that all values of y are passed, also y = - 8.
