I have to find some level curves for:

So, if we call at the surface given by the equation , then

Now, that particular case its simple, cause it gives just a point, but if I go downwards I get:

I'm not sure how to represent this. How does this look on the xy plane?

I know that:

And

But it don't helps me to visualize the "curve". I know actually that it looks like a parallelogram, but thats because I've used mathematica to compute the surface :P I don't know how to deduce it analytically.