1. The problem statement, all variables and given/known data 2. Relevant equations (1) (pink) Is it vertical displacement ? * 4 - 2 (unstretched lenght of the spring) ? (2) (blue) What does it mean ? * I was thinking that maybe they used The Pythagorean theorem (62+42)=7,2 ≅ 7) but I'm not sure. Besides I don't understand why they used it. 3. Attempt My reasoning: In general I think they are using this formula, where in that case the left side is the initial state - the right side - the final state of the spring.