Ok, so, i have a serious problem and i need help, since english is not my mother language i'm not sure if this is the right name of the problem..

i have function in implicit shape y^2 = x^2 * ((4+x)/(4-x))

and for the curvature there is formula http://img203.imageshack.us/img203/727/formulaq.jpg

and i don't understand how to get to those parameters

help pls