If you know the minor scale, the root of the relative major is always the third note of the minor scale. For example, “F” is the third note of the D minor scale (D, E, F, G, A, Bb, C). F major is the relative major of D minor. It works the same for all minor scales.
Answer: A memoir is a group of past memories from that person. Though an autobiography is the actual story of that person's life. A biography is a look at someone else's life/story.