Learn Farsi: جواب او درست نیست.

To ask for confirmation, you can add مگر نه؟ (magar na?) or این‌طور نیست؟ (in-tor nist?) to the very end of the sentence. You would say: جواب او درست نیست، مگر نه؟ (javāb-e ū dorost nīst, magar na?). In Farsi, you can use these exact same tags whether the main sentence is positive or negative.

I see the word او (he/she), but how does it mean "his" here?

Farsi shows possession by linking the noun and the pronoun with a short "e" sound called the ezāfe. Here, جواب (javāb - answer) connects to او (ū - he/she) to make جواب او (javāb-e ū), which translates perfectly to "his answer".

Could this exact sentence also mean "Her answer is not correct?"

Yes, absolutely! The Farsi pronoun او (ū) is gender-neutral, so it means "he" or "she" depending on who you are talking about. Without any other context, جواب او can mean "his answer" or "her answer".

I've learned the word غلط (ghalat) for "wrong." Can I use that instead?

Yes, you could completely swap it out and say جواب او غلط است (javāb-e ū ghalat ast), meaning "His answer is wrong." Saying درست نیست (is not correct) means the exact same thing but is just a slightly softer way to phrase it.