This should be a simple one:

I know that the formula is

$\displaystyle \frac{x-x_{0}}{A} = \frac{y-y_{0}}{B}=\frac{z-z_{0}}{C}$

so long as A, B, and C are all non-zero numbers. But what if one of the direction numbers is zero? Do you just drop that part of the equation since it is undefined? For instance, if C is 0, then

$\displaystyle \frac{x-x_{0}}{A} = \frac{y-y_{0}}{B}?$

Or is the whole equation undefined?