Evidence and sound reasoning.
A good answer is always supported by facts.
Ex: if you're writing a poem commentary, your rational must include lines and words as support
Ex: If you're going an history project, you must use historical dates, different sources, etc
Ex: If you're going a science research project, you must always support your answer with observations/test results.
The length depends on the subject, topic, and question so there's no answer to that.
Also, like Ghost said, a "Good" answer is relative to the teachers or popular viewpoint. What one person thinks is good is not necessarily the viewpoint of another.
I have outlined two things what should guide you towards a "good" answer.