In this tutorial learn about the meaning of undefined slope. This is also the case for all the lines above. Every point on the line has x coordinate 15 thats why its equation is x 15.
The equation of a line in general form with a point -3 0 and an undefined slope is x 3 0.
In fact this is a special case and we use a different equation like this. We know that slope of a vertical line is undefined. In this case the line passes through -2-6 and therefore the constant is the value of the x-coordinate. As a vertical line will always make an angle the size of of 90 with the positive direction of the x -axis.