Hey it's not gone, but this mark you got needs to be improved immediately (I.e. You can't bomb out the next assessment). This mark you received you need to ask yourself some serious questions such as did you study enough?, did you do practice papers? Or simply you don't understand maths 2 unit. You MUST identify the reasons why this happened and then there's no easy way around it but you have to work your ass off in these Christmas holidays to improve your results. You need to first off start using textbooks and then progress onto doing exam questions. Getting a tutor for maths that is 1:1 would also be helpful. On the other hand it might be worth considering moving courses to general maths. Although you might think you are going to get scaled bad, getting a 85 HSC mark in general maths is much better than getting a band 3 in 2 unit maths. From here what happens is up to you, you can keep worrying about your atar or actually do something to fix it, best of luck