@jemidiah Yes, as I stated in my post, since the rectangle is in the X-Y plane any normal will have only a k-component. I did not mean to imply that there is only one normal either. I know it's a trivial example but it does show the steps and it can be extended to 3D very easily where the result is not so trivial. There are applications, like say 3d-graphics, where the direction of the normal (even with objects that only lie in a plane) is important and one would have to program the computation process.

I guess the problem is that we don't know what sally is using this for (well at least i don't). I was hoping that my post would encourage some input from her to get what she is after.