ok, so what i've gathered from your ramble, here are you options (correct me if im wrong):
1. keep all subjects + take up Ext 2 = 12 units
2. drop Biology + take up Ext 2 = 10 units
3. keep Biology + Ext 1 = 11 units
4. keep Biology + drop Ext 1 = 10 units
If you're getting 90% in Biology...