No answer is possible using only the information given in the question. In addition to the angle of depression, we need either one of these: -- the length of the tree's shadow on the ground -- the slant distance direct from the tip of the shadow to the top of the tree.
I'm going out on a limb and guessing that on the page, next to that much of the question, was a picture of the tree, with one of these additional items labeled. There's an important reason why the publisher decided to go to the effort and expense of printing the picture right there. I leave you to ponder the reason.
