A 5.0-kg red book is suspended from a spring attached to the ceiling of an elevator that is accelerating downward. Attaching the red book from the spring causes the spring to stretch 71 mm beyond its relaxed length. You also have a yellow book, whose mass is unknown. 

What you do know is that when you attach both the yellow and the red books to the spring together, the spring stretches 110 mm beyond its relaxed length. You also know that when the elevator is at rest, the spring stretches by 140 mm beyond its relaxed length when both books are attached to it at the same time. 

You want to find out: 

a) What is the spring constant? 

b) What is the mass of the yellow book? 

c) What is the elevator’s acceleration?