/sci/ - Science & Math

I am working out of Bak and Newmann by myself and trying to do each problem that has a solution in the back for feedback purposes, although idk how beneficial that really is. However, I do not understand how they came to this answer and I think what bugs me about it is how they wrote the equation to the plane where it is stated differently in the text prior to Proposition 1.12. I tried to include all the necessary information here.