1. **State the problem:** We need to find the height of a tree that casts a shadow 21 meters long, given the angle of depression of the sun to the tree is 51°. 2. **Identify the right triangle and trigonometric function:** The shadow forms the adjacent side to the angle, and the height of the tree is the opposite side. We use the tangent function because $ \tan(\theta) = \frac{\text{opposite}}{\text{adjacent}} $. 3. **Write the formula:** $$\tan(51^\circ) = \frac{\text{height}}{21}$$ 4. **Solve for height:** $$\text{height} = 21 \times \tan(51^\circ)$$ 5. **Calculate the tangent:** Using a calculator, $$\tan(51^\circ) \approx 1.2349$$ 6. **Multiply to find height:** $$\text{height} = 21 \times 1.2349 = 25.933$$ 7. **Round to the nearest tenth:** $$\text{height} \approx 25.9 \text{ meters}$$ **Final answer:** The height of the tree is approximately 25.9 meters.