This mission is stupid. Start with the file, untar it:
$ tar xzvf stego14.tar.gz 6578747261637400.jpg
If you look really carefully at that "jpg" under a hex editor you may notice that is is also a rar.
$ cp 6578747261637400.jpg file.rar $ unrar e file.rar Extracting key.jpg OK All OK
So you have 2 things now, the Cipher text of PGNNZCFYXD (yes that is a D at the end, not 0) and a key of (*5+10). If you are clairvoyant you might notice that this is an affine cipher. Input this in a decoder: Link here
You will get "bulldozinj" (not bulldozing, that is a red herring)