Delete Partition on Mac

Hello Guys,

I am facing a strange issue and one that I tried researching on Google but all the solutions failed.

I used Apple's bootcamp and installed windows. For that, I created partition. I'm done with the windows task and now I want to restore everything as before. I erased the partition but I'm unable to merge the storage capacity into the original macintosh volume. I researched on internet where various articles suggested a simple trick of clicking on the 'minus' button to remove it or clicking on the partition button and remove the partition, but in my case both the options are unavailable.

Here's what I'm seeing:

Please help me!