Losing disk space on my MBA

Avicenna said:

I've lost disk space on my hard drive. Take a look at the disk utility screenshot below, it shows it's size is 83GBs but the hard drive size is 121. Also, every time I delete something, I only get a little bit of space back:

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Click to expand...

It looks like your drive has one partition that doesn't take up the entire drive (as in you have one partition that's set up for 83.96gb and the rest of your drive is unformatted). See the little lines in the bottom right corner of the blue part of your screenshot? That indicates where the partition ends. On my drive (attached below), that goes all the way to the bottom which indicates the whole drive is one partition.
The blue shading shows you how much of the drive is full (so in my case, my drive is almost half way full). You partition looks all the way full. You may need to make a Time Machine backup and repartition the drive to take up the whole thing.