the lessons do not distinguish options Google says mimi kujibu is correct in other words, what is the word, prefix and pronoun is never clear in many examples. This seems to be a major problem in this language program. Also, the lessons give many words with similar meanings but is not clear on when these are to be used. Swahili seems to have grown like Yiddish with a real mix of grammar, like kinyarwandan
Lesson number one: Don't trust Google! Mimi kujibu is wrong.
Have you looked through the tips and notes? If you had, you'd know that ni- means "I" and -na- indicates the present tense, leaving -jibu to mean "answer/respond". Because the ni- indicates "I", you can drop the mimi, but you can add the mimi to emphasis I.
The ku- in kujibu indicates the infinitive/gerund form of the verb, so mimi kujibu is just "I/me ... to answer"