Fix, six, xenix, appendix.
thorax
Flax.
NONE!
cortex
Quincunx is a word. It ends with X.
phoenix?
Five letter words starting with C and ending with X are calyx and codex.
The word faux has a silent x, pronounced [foh].
Pheonix
A four letter word beginning with C and ending with X is crux.
Try flux or flax; latter is healthier.
The most common word of that form is "relax". Others are "radix" and "redux".