"Why and how?"
Translation:Mengapa dan bagaimana?
"Mengapa" is preferred in formal writing, while "kenapa" is used freely and more commonly in everyday exchanges and informal speech.
"mengapa" is the formal style ("baku"), it's the form that should be used in formal writing.
"kenapa" is informal, it's better not to use this in formal writing, but it's absolutely fine to use it in daily speech.