4:56 AM
its easy to be frustrated at exams but I don't know a better alternative. Reading it again, it is possible (given the context of the syllabus, which I do not have) that the "differential equation of a curve" (which is not standard notation) means that it needs to characterise the family of curves
you're given finitely many choices so you can just check which ones it could be for a few choices of b, maybe b=1 is enough
probably and more helpfully I would guess you're supposed to differentiate once and then eliminate b from resulting system of two simultaneous equations