Partition Tab in Disk Utility is Greyed Out

Question

Basically I wanted to try out Mac OS X Yosemite, so I created a partition on my hard drive. Now, I am trying to remove this partition, because it is no longer needed, but Disk Utility is simply not allowing me to.
I heard there may be a way to solve this using Terminal. Is that possible?
Any answers and/or helpful comments is appreciated in advance. Thank you.

Answer 1

And, this was from the Internet Recovery utilities window, right? OK, this next suggestion depends on which MBA model you have. Check the link below. If your model came with Lion pre-installed then it is capable of using Internet Recovery. If it came with a late version of Snow Leopard it can be upgraded via a firmware update. You will find this in the link below.
Computers that can be upgraded to use OS X Internet Recovery
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
Boot to the Internet Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND-OPTION- R keys until a globe appears on the screen. Wait patiently - 15-20 minutes - until the Recovery main menu appears.
Note: You will need an active Internet connection. I suggest using Ethernet
            if possible because it is three times faster than wireless.
Once the Utility Menu appears select Disk Utility and click on the Continue button. Now try removing the partition again. If you have the same error, then do the following:
From the screen's menubar select Terminal from the Utilities menu. Enter the following at the prompt:
     diskutil list
Press RETURN. You should see a display of your devices. Next enter:
     diskutil unmountDisk force /dev/disk0
Press RETURN. Now, if this works your internal drive should be unmounted. Return to Disk Utility and try the process of removing the partition.