conics equation (1 Viewer)

[elliottjohnson]

you are given e=2/3 and directrices x=+or-9
how do you find the equation. why isnt the answer x^2/9+y^2/5 = 1

instead it uses the directrices to get x^2/36 + y^2/20 = 1

how do I use laTex when posting a new thread??

thanks!