Learn Arabic (Modern Standard): ما سعر الإطار؟

This is the core rule of the Idafa (possessive) construction! When you link two nouns to say "the X of the Y", the first word (سعر) never takes the word for "the" (ال). It is automatically understood as specific because it is attached to the second word.

How do we know it means "the tire's price" and not just "a tire's price"?

In an Idafa construction, the definite article ال (the) on the second word, الإطار (the tire), makes the whole phrase specific. If you wanted to say "a tire's price" or "a price of a tire", you would drop the ال and just say سعر إطار.

Since the translation is "the tire's price", how would I say "the price of the tire"?

You just said it! In Arabic, the Idafa structure سعر الإطار literally means "price [of] the tire". It serves perfectly as both "the tire's price" and "the price of the tire" since Arabic doesn't use a separate word for "of".

Why does this sentence use ما (ma) for "what" instead of ماذا (matha)?

In Arabic, ما is used when asking "what" before a noun (like سعر, meaning price) or in equational sentences without a verb. ماذا is used when asking "what" before an action/verb (like "What did you do?").

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