affix fix complex flex apex appendix ax box
The most common word of that form is "relax". Others are "radix" and "redux".
Examples of common nouns ending in X:axboxcrucifixcruxduplexequinoxflaxfoxhexibexindexjinxlarynxloxlynxmatrixminxmixonyxoryxoxparadoxphoenixphloxpoxprefixrouxsexsixsmallpoxspandexsuffixsyntaxtaxthoraxvortexwax
thorax
NONE!
Flax.
cortex
Quincunx is a word. It ends with X.
Fix, six, xenix, appendix.
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