Question 3
You are not requested to come up with a real life example, but rather with a schema and a set of tuples - e.g., (a1, b1, c1), (a2, b2, c2) - that violates the "rule".
Question 5
You cannot assume item# is unique; you also cannot assume there is only one item# with the same color or size.
Question 6
Note that the original algorithm to compute the new set of FDs is described in the book, page 147.