Thanks for the tip, elkka!
That made a big difference in the quality of my results - in every single case, my "b"'s are perfectly aligned. Unfortunately, it also made a big difference in the answer I submitted
It's a bit annoying that a technical, implementation package-specific tweak can have such a big effect and cause one to submit the wrong answer when one has otherwise done all the correct setup and formulation of the problem, but I suppose this is yet another example of the challenges in designing homework problems for an online audience.
The exercise was definitely worth it, I learned quite a bit from this problem, not the least of which is how to use a program like Octave - and as my wife has to keep reminding me, I'm not actually getting a *grade* in this class...