Greg's right. The formal version of the word "you" (Sie) is always capitalized (in all grammatical cases, so "Ihnen" and "Ihrer" as well). Not so with the word "sie" meaning "they". In this sentence, that little distinction is your only clue that this is "you" and not "they".
Look at the verb ending, then context.
- Sie kommt = she comes
- Sie kommen = you (formal, singular or plural) come or they come
For the second sentence I gave, you need context to tell if "Sie kommen" means you or they. Duolingo never (99% of the time) has any context so they almost always accept both you and they as English translations.