Based on the information in the problem alone, the height of the building has little to do with the height of the tree. It could be 1200 ft, or 20 ft.
However this could be a similar triangle problem, with more information, in which case the drawing would look something more like:
Can you post the exact wording for the problem you're working on? Based on the above there are multiple solutions.A girl wants to find the height of a building. She is standing 393 feet away from the building. There is a tree 42 in front of her, which she knows is 15 feet tall. How tall is the building?
"Ilse wants to find the height of the tallest building in her city. She stands 393 feet away from the building. There is a tree 42 feet in front of her. which she knows is 15 feet tall. How tall is the building ? ( Round to the nearest foot.)
It's not your fault, the question is poorly worded. If your teacher doesn't often give "no solution" or "all real number" problems, then I'd assume this is a Similar Triangle problem, and the diagram I drew above is correct.
Solve for x:
However, if this were an ACT/SAT question, the answer would probably be something to the effect of "not enough information."