I was working through some of the problems in the Wolfram Problem Generator, specifically "solving systems of four equations", and I was given this problem that had the variable 'u' in it. When I entered a solution, all of the other variables input were interpreted correctly, but next to 'u' it says "(no solutions) u (unified atomic mass units)". Here's a picture: Yes, I know I solved it incorrectly, but the question it generated for me prior to this one had a 'u' in it as well and I solved that one correctly, but it still said I was incorrect due to improperly inputting the variable 'u'. How do I properly input the variable so it won't read it as unified atomic mass units?