Answer:
The hyperbola has two directrices, one for each side of the figure. You can see the hyperbola as two parabolas in one equation. So, as parabolas have directrix, hyperbolas does too.
The directrices are perpendicular to the major axis. That means if the parabolla is horizontal, then its directrices are vertical, and viceversa.
Therefore, to find the right line that forms the directrix of a hyperbola, you just need to use the directrix that is perpendicular to the major axis.
