If you ask strictly about partitions, then you can't...
One partition is on one disk, the other partition is on another disk. Therefore, you cannot combine/merge both partitions. One part(ition) of a disk cannot be at the same time also be a part(ition) of a separate disk.
(That is, unless you would utilize some disk controller or low-level storage device driver which presents the two separate disks as a single logical disk to the operating system, like RAID0 or Windows Storage Spaces, for example; but i digress and that would also not preserve your existing partitions...)
However, what you could perhaps do is creating a (directory) junction on drive C:, which would refer/link the root folder of the 2nd/yellow partition. (The 2nd partition would need to be formatted, of course.)
To create a junction, use a cmd console shell with admin privileges and type:
mklink /J <junction_name> <existing_target_directory>
<junction_name> is the path and name of the junction to be created. For all practical purposes, the junction will appear and function like a directory.
<existing_target_directory> is the full path of the directory the junction will refer/point to.
mklink /J C:\foo\bar F:\
will create the junction "bar" in the directory "C:\foo", with the content of the root directory of F: appearing in "C:\foo\bar". CD'ing/navigating into "C:\foo\bar" makes the content of "F:\" appear as if it were the content of the directory "C:\foo\bar".
In other words, if there is for example a file
F:\baz.txt, it could then also be accessed by the path
C:\foo\bar\baz.txt. And if you were to create a file (or directory)
C:\foo\bar\somedir\file.dat, it would not be stored on the partition of drive C:, but it would be stored on the partition of drive F: (with its "true" storage location being