I need to figure this out before I take my test! HELP!

Here's a link to a picture of the graph: http://i14.photobucket.com/albums/a3...Picture2-9.png

I'm pretty sure a=3, but I'm not sure how to figure out a or c, or the final equation?

My choices are:

a) x^2-y^2=9

b) y^2-x^2=9

c) x^2+y^2=9

d) x^2-y^2=3

How do I figure this out?!

THANKS.