Forgot my geometry (Revived)

[Shaggy Hiker]

I think I'm overlooking something simple, but I figured somebody here would have a ready answer. Here's the problem:

For a line A-B with point C on the line halfway between A and B, such that AC = BC, and a point N not on the line, what is the formula to find the angle ACN given only the lengths AB, AN, and BN?

Basically, I have two sensors that will be picking up a return echo off of an item that will be somewhere in front of it, and I need to find the bearing to it. The purpose of the point C is that the bearing from either "eye" is not all that meaningful.