"Undefined" slope means the line on the graph is vertical, and that tells the whole story right there. The 'x' coordinate is the same for every point on the line. If (6, -1) is a point on the line, then you know that the x-value of EVERY point on it is 6 .
It's a vertical line that intersects the x-axis at x=6, and the equation of the line is x=6 .
