1. **Stating the problem:** We have three squares arranged vertically between two parallel horizontal lines. The top square is rotated so that one corner touches the top line, forming a 15° angle with the line. The middle square is rotated with an adjacent angle of 42°. The bottom square is rotated so that one corner touches the bottom line, forming a 35° angle with the line. We need to find the angle $\alpha$ between the middle and bottom squares. 2. **Understanding the setup:** Each square has right angles (90°) between its sides. The angles given (15°, 42°, 35°) are the angles between a side of the square and the horizontal lines. 3. **Key fact:** The angle between two adjacent squares is the difference between their rotation angles relative to the horizontal lines. 4. **Calculate the rotation angles:** - Top square rotation angle: 15° - Middle square rotation angle: 42° - Bottom square rotation angle: 35° 5. **Find the angle $\alpha$ between the middle and bottom squares:** $$\alpha = |42° - 35°| = 7°$$ 6. **Conclusion:** The angle $\alpha$ between the middle and bottom squares is $7°$.