[seqfan] Congruence question

[andrew at nevercenter.com]

While working on some sequences, I've run into congruences that look like
this:

x^2 + ax == y^2 mod (b)

where a and b have a known factorization. Are there a.) any "easy"
(computationally speaking) ways to find solutions for this kind of
congruence, and b.) does knowing the factors of a and b help at all?
Thanks!

    -Andrew Plewe-