Essentially, the optimal outcome would be to rank as high as possible relative to your cohort as this would maximise your chances of receiving a high Assessment Mark, being the mark that reflects your internal performance and contributes 50% of your final HSC mark for a subject (in this case, Modern History). The marks that you receive for your school-based assessment tasks/exams, like the 84% you received for your assessment task, are only used to determine your rank and their significance does not extend beyond this.
If your mark places you second last in your cohort, this could indicate either one of two things:
- Your cohort is comprised of academically capable students - If this is the case, then it is possible that your current position may not be as bad as you might think, although improvement in your upcoming tasks is definitely welcome as it would assist you in ranking higher and consequently brings you closer to your ATAR goal.
- Your school sets easier/relatively easy assessment tasks/exams, making favourable performance in the actual tasks easier to achieve - If this is the case, then it is important that you focus on improving your performance in your upcoming tasks. From your post, it seems like this you can definitely do this, especially considering that you misunderstood the question (which you can hopefully address going forward) rather than a more concerning issue.
With that being said, 30% is not insignificant, meaning that it is reasonable to suggest that it will impact your overall position in Modern History, although this does not necessarily mean that a 90+ ATAR is no longer achievable, especially considering that there is 70% worth of your school-based assessment tasks/exams that is yet to be determined, giving you a great opportunity to turn things around in your favour.
I hope this helps!