I hope its not too late to help. Cause when i picked 2 Unit, i was very good a Maths in year 10 and earlier. But too often i ignored the homework assuming it would as easy as year 10, but i was very wrong. This lead to me failing the entire preliminary course. I am year 12 and still currently doing 2 unit. I have regained my grades and doing fairly well, this though was a result of some seriously hard work and constantly studying and doing homework. You can do really well in 2 unit as long as you care about your work and do homework. DO NOT leave gaps in your understanding of mathematical concepts.