Rid polynomial of square-root

Hi,
Starting with:

   x = sqrt(x^2 + 1) - a

How do i get it into the form:

   x^2(b^2 - 1) + 2*a*b*x + a^2 - 1 = 0


Also, i tried to solve for x:

                     2
(%o1)     x = sqrt(x  + 1) + a

(%i2) solve(%, x);


It just gives:
                     2
(%o2)    [x = sqrt(x  + 1) + a]

I've read all the manuals.

-- 
regards,
Russell Shaw