Answer:
i. Your backyard is a Garden of Eden refers to the Old Testament Book of Genesis. It is a Biblical allusion, it refers to your backyard as the paradise God made for Adam and Eve.
ii. You're a regular Einstein means that your intelligent and smart. It is a historical figure allusion.
iii. Don't be a Scrooge. Refers from Charles Dickens, A Christmas Carol book. It means don't be mean because Scrooges are mean.
iv. That new business of his is going to sink like the Titanic! It refers to the movie about the Titanic and a historical event. Means the new business is going to sink/fall.
v. Potato chips are my diet’s Achilles heel. It means potato chips are my weakness. Refers to Achilles in ancient Greek mythology.