cc /xs/ and xc /xs/

The cc/xs/ & xc/xs sounds 

Often, the sound is pronounced "xs" but can be spelt as "cc" or "xc" but depends on the opening vowel. It is still the same sound but there are two ways to spell the word when using this sound.   

For example, accelerate is "cc" but exceed is "xc".

One helpful rule to remember when spelling words with the xs sound:

In a word, the letter "x" will never be followed with an "s" so even though it's a "xs" sound, the spelling will only be cc or xc but never xs. 

The following video link will help you to understand this rule.