Given that the vertical and east direction are perpedicular paths, you can construct a right triangle with 120 feet and 280 feet as the legs.
The distance between the balloon and the site where it was released will be the hypotenuse of such triangle, so you can use Pythagorean theorem to find the distance.
c^2 = (120ft)^2 + (280ft)^2 = 78400 ft^2
=> c = √ (78400 ft^2) = 280 ft
Answer: 280 ft
