Solved 8GB RAM but only 6GB usable

Guys, I need your help because my usable memory is just 6GB out of 8GB after I installed Android SDK and reserved 2GB of memory for the Android itself. Now I want to go back to 8GB by just unistalling the Android SDK but it didn't work.

I don't see your problem. The memory graph clearly shows that there is 7.9 GB usable and other numbers are consistent with this. There is 3.0 GB in use and 4.8 available with a sum of 7.8 GB. The difference is due to rounding.

Oh, okay. I thought the 'available' one means how much the memory is usable. Thanks though.