Write the equation of the line with x-intercept (–6, 0) and undefined slope.

would this be wrote as

0=k*-6

I would write it as y= k(x-6)

an undefined slope means that the line is vertical, there is no y term in your equation, since you are not "describing" the y value.
all you have to "describe" is the x value, so it would simply be x = -6

similarly a horizontal line would say y = b, where b is a constant.

Reiny,

Can you explain this in greater detail?

I so not understand.