When Do You Pronounce The Letter C As S Or As K?

There aren't many 100% rules, but the following should help:

"C" normally has a /k/ sound if it somes before the letters a, o or u. (cat, cot, cut..) If it comes before e or i (or both) it is usually /s/ (centre, city.) This difference still applies no matter what comes after the initial letter. For instance, if you take a word like coin or cue, the sound is still /k/, based on the first vowel (o or u) - the fact that the NEXT letter is i or e makes no difference.

"C" is nearly always pronounced /k/ before a consonant. There aren't many  consonants that will go after a c anyway - the ones you can use are l and r (clear, crash). The exception to this is h - if h is used after a c, it produces a /ch/ sound, as in church.

So c+ consonant never produces an /s/ sound.