Placing one detail on another

Hello.

I have a problem with two details. I can not place one on another like I can do with my hands in reallity(See the attached photo).

One detail is bigger than another.

Details connect with three points.

Point A and Point B moves from thorn with the same distance. But what about point C? How should I find the plane of second detail if I have just first one and two intersection points?

I am willing that somebody will help me to solve it.

Any help would be great.

Thank you in advance.

Aiste