1. **State the problem:** Find the horizontal distance from the airplane to the landmark given the altitude and angle of depression. 2. **Given:** - Altitude (vertical side) = 2000 m - Angle of depression = 78° - Horizontal distance = unknown (let's call it $d$) 3. **Diagram and triangle setup:** The airplane is at height 2000 m above the ground. The angle of depression from the airplane to the landmark is 78°, which means the angle between the horizontal line from the airplane and the line of sight to the landmark is 78°. 4. **Use trigonometry:** In the right triangle formed, the altitude is the opposite side to the angle of depression, and the horizontal distance is the adjacent side. The tangent function relates opposite and adjacent sides: $$\tan(\theta) = \frac{\text{opposite}}{\text{adjacent}}$$ So, $$\tan(78^\circ) = \frac{2000}{d}$$ 5. **Solve for $d$:** $$d = \frac{2000}{\tan(78^\circ)}$$ 6. **Calculate $\tan(78^\circ)$:** Using a calculator, $$\tan(78^\circ) \approx 4.7046$$ 7. **Calculate $d$:** $$d = \frac{2000}{4.7046} \approx 425.3$$ 8. **Answer:** The horizontal distance to the landmark is approximately **425 metres**.