Translation:Mathematics does not have to be boring.
The problem here springs from an idiosyncrasy of the English language, namely that "must not" is not the negation of "must".
Compare the sentences: 1 You mustn't throw stones. 2 It is not true that you must throw stones.
Polish on the other hand treats the negation of musieć "correctly" (in agreement with propositioal logic).
OK, if that is the case, then how can one say "Mathematics cannot be boring." I would say "Mathematics must not be boring," for instance, to a bunch of young math teachers, exhorting them to make it interesting for their students, while I would say "Mathematics cannot be boring" (no matter how hard I might try to make it) to students, because it is so foundational to so many interesting things.
I always forget "powinnac" (if that is the infinitive). It's not quite as forceful as "music," though, is it. This is one of the hardest things for me in Polish, because my first Slavic language was Russian, which often translates pretty nicely into Polish, but must and should work quite differently.