The situation is represented by a drawing with a right triangle where an airplane is at the top corner, the elevation angle (opposite to the airplane) is 70°, the height (vertical leg) is x, and the adjacent leg (horizontal leg) to the 70° angle is 800.
Then, to find the height x, which is the opposite leg to the angle, you can use the tangent ratio, which is opposite leg divided by adjacent leg:
tan(x) = opposite leg / adjacent leg => tan(70°) = x / 800
Answer: tan(70°) = x / 800