I have 3 sets of X,Y,Z axes which differ by rotations only. Call them A, B, and C.

In A, there is a unit vector (0, m, n). The same unit vector relative to A exists in both B and C as (bx, by, bz) and (cx, cy, cz).

I need to find the angle between the X axis of B and the X axis of C as projected ("flattened") onto A's XY plane.

It's been 35 years since my last linear algebra class, so any clues would be greatly appreciated!

John