The first one is correct in formal and informal contexts. The second sentence would not be used by a native speaker.
The reason is that there is a difference in the way that English handles wh-words, when they are in the main clause vs. when they are in the embedded clause. The structure also depends on whether the wh-word is the subject or the object in its clause.
In the main clause, we use the basic order: wh-word verb ______.
Where is the best place to ask this question?
This is true when the wh-word is the subject or the object, although you'll notice we use do-support when the wh-word is the object (when the main verb is not is):
- Who told you that? (who is the subject, no do-support)
- Who did you tell? (who is the object, use do-support)
When the wh-word is inside of an embedded clause, the verb doesn't move to the front of the clause; only the wh-word moves. For example:
- I was wondering [who told you]? (corresponds to "who told you?" in main clause)
- I was wondering [who you told]? (corresponds to "who did you tell?" in main clause)
(In other languages, this pattern is not the same; it is an arbitrary feature of English.)
Since where is the object in this case (and usually is in general), it follows the second pattern, so: "I do not know where ... is".
The copyeditor in me observes that your at there adds nothing to the sentence: leave it out.
It’s also something of a shibboleth whose use will mark you as “poorly educated”, because these things are expected to have been corrected in the course of proper schooling. The same is true with questions like “Where are you at?” Here again that at at the end adds nothing — or at least, nothing good — and should be omitted.
Once upon a time, surely aeons ago now, there was something of a hipster phrase about “where it’s at”, referring to some location or event or even style that was supposed to be “really happening”. Unless you’re trying to faithfully recreate the special slang from that period in history, I would avoid it.
Best Answer
The second one is not strictly grammatical, for the reason that it repeats the interrogation of the sentence.
I can say Where is the bank? or Could you tell me where the bank is, please?.
Both of those are questions. But if I say Could you tell me where is the bank, I am placing a double interrogation, in the forms of Could you and where is. So it tends to sound awkward in English.
Having said that, it is a mistake that non-native speakers frequently make. I have particularly noticed it in the speech of other Europeans. Because it is a small error, and everyone understands you, people will not normally correct you. That may be one reason why the mistake persists.