Learn Turkish: Şirketiniz nerede? - Where is your company?

It comes from şirket (company) + -iniz (your). The suffix -iniz is the possessive ending for "your" when you are talking to multiple people, or when you are speaking formally to a single person.

Why does the possessive suffix use the letter i (-iniz)?

Possessive suffixes follow 4-way vowel harmony. Because the last vowel in şirket is e, the suffix must use i. If the word had a different vowel, like the a in araba (car), it would become arabanız instead.

Where is the word for "is" in this sentence?

In Turkish, the "to be" verb for "is" or "are" is often invisible in the present tense for the third person (he/she/it). Şirketiniz nerede? literally translates to "Your company at-where?", and the "is" is simply understood.

Why don't we use a question particle like mi here?

Question particles like mi or mı are only used to form "yes/no" questions. Since this sentence already contains a specific question word (nerede, meaning "where"), adding a question particle is unnecessary.