I suppose you mean to have a third variable, z, in that expression? If so then you should be able to follow the same reasoning starting at 17:15 in the video. The surface won't be a sphere, but something similar. I recommend looking at how it intersects the coordinate planes (xy-plane, yz-plane, and xz-plane). You can do that by setting one variable equal to zero and looking at the curve defined by the remaining two variables.
If you mean the values of c we chose here, there's no restriction on what those might be in general But, for specific problems, some values will be more appropriate than others. You will need to rely on your intuition and experience to choose the best values of c. Sometimes it's just trial and error.
