[{(x-y)^2-z^2}/{(x+y)^2-z^2}]*[(x+y-z)/(x-y+z)]
you may want to look at this
http://www.wolframalpha.com/input/?i=[{(x-y)^2-z^2}%2F{(x%2By)^2-z^2}]*[(x%2By-z)%2F(x-y%2Bz)]
doesn't look like its going to simplify too much... that is if OP is correct..