Without a diagram, this may be tricky to seem but the shallower angle of depression (below the horizontal) is associated with the far end of the island, and the steeper angle of depression with the near end. You can form two right triangles by drawing lines from the plane to the far and near end of the island, and then drawing a line straight down from the plane to the ground. The two lines to the island ends are the hypotenuses of these two triangles, and the line straight down is one leg (that is the same for both triangles). In this drawing, the other legs of the triangles are drawn from the spot on the water directly below the plane horizontally to each end of the island, respectively.
I know this may seem confusing without a diagram, but the difference in the lengths of those two horizontal sides of the triangle constitutes the length of the island. If we can calculate the lengths of those horizontal sides of the two triangles, we could them compute the length of the island.
If you draw out the triangle, the horizontal sides are related to given quantities by:
tan(90-θ) = (horizontal side)/(altitude of plane)
where θ = the depression angle below the horizontal for each triangle. This is because, for the angle (90 - θ) in each triangle, the horizontal side on the ground is "opposite" that angle, and the line drawn from the plane to the water directly below (whose length equals the altitude of the plane) is "adjacent."
The (horizontal side)far end = (altitude of plane) * tan(90 - 43) = 2850 m * tan(47) = 3056 m
Do the same for the other end of the island, and then subtract the smaller number from the larger, This will be the length of the island.
Be sure you can explain the geometry to yourself in a diagram. If you would like help with this, just let me know, and I can meet with you online to discuss it,
