Which phase of a writing strategy involves deciding on the topic and the purpose of the essay?

Well, from my experience, it would be prewriting. Prewriting is where you come up and brainstorm ideas. Its when you scribble ideas and write out a plan on the direction that you want your paper to head to, where you would write a list or draw a picture on key points you want to hit on the paper. 

Ibelieve 8 + 2 = 10 sorry i'm not good at math but we can be friends : )

Narrative poetry is a form of poetry that tells a story, often making use of the voices of a narrator and characters as well; the complete story is usually written in measured verse. narrative poems include epic ballads, idylls and lay people. lyric poetry is a type of poetry similar to an emotional song, which is distinguished from dramatic and narrative poetry.