Draw a picture----always, for this kind of problem.
There is a right triangle: One side is the distance from the ground to the balloon. Another side is the horizontal distance to the landing spot. The longest side is the hypoteneuse, which is the actual line-of-sight distance from the balloon to the landing spot. (this is sometimes called "slant range")
You are given the horizontal and vertical distances--which are the same, meaning that the tangent of the "angle of depression" is 1.