How to delete an unwanted hard drive partition in OS X El Capitan?

Question

Hard disks

Friends, in Disk Utility there is no way to delete an unnecessary hard disk partition in order to later merge it with the main one. What am I doing wrong?
Screenshots:
1. So to speak, a general photo:

2. This section is now the main one:

3. Here it needs to be deleted:

4. The delete button is inactive. Actually why? Through OS X Recovery, the same thing.

Previously, this section was used as the main one. Installed a clean OS on a Macintosh HD yesterday. Today I formatted the old partition (Data), but I can't delete it.

Answer 1

Partition resizing is active, so what's stopping you from resizing the desired partition? You reduce the size of data to 0, then you increase the size of the one you need due to the empty space.