Ok, my thoughts on your problem;
You say that General Mathematics is your poorest subject as you don't listen because it is too easy - perhaps the best solution to this problem is to listen a little more to imporve your marks! If it is easy, then from my point of view you should kept it as you can get easy marks from it.
But, if you don't enjoy it and are really just wasting time sitting there in the class then i suggest that it would be better perhaps to drop it. From what you say you already have a fairly heavy workload with 4 Unit English and your other subjects, and this time could be spent on your other subject raising those marks as you say.
I agree with mazza though, you need to take into consideration what you want to do after school and if Uni is the way then what sort of UAI you need. 10 units is the minimum for a UAI, so this can be a risky business since every one of your marks would count...whereas with 12 units you can relax a little bit.
Now don't quote me...but one of the girls from our school who is doing 3 Unit English repeated something about how she wasn't going to pick up 4 Unit because no matter what your marks in the subject, only two units worth would count towards UAI. I don't know how reliable that info is, but i would talk to your teacher.
I will stress however, that no matter what any one of us says here, the decision is up to you and it is your future! Talk to your teachers/year advisor/careers advisor/mentor...someone that knows you, cause they will be able to give you a far better idea of how they think you'd cope & whether it is the best option for you.
Good luck with it. I know how hard is it to decide cause i'm trying to decide whether or not to drop back to 4 unit mathematics at the moment.