Answer:
x = 6.
Step-by-step explanation:
The general form is r = ep / (1 + e cos Ф)
where e is the eccentricity, p is the distance of the directrix from the focus.
Changing our formula to the above we divide top and bottom by 4:
r = (26.4)/ 4 / (4/4 + 4.4/4 cos Ф)
r = 6.6 / (1 + 1.1 cos Ф)
e is > 1 so we have an hyperbola here.
e = 1.1 and ep = 6.6
giving p = 6.6 / 1.1 = 6.
The equation of the directrix is x = 6.
