You need to use tan since the problem requires u finding out the distance from the airport and ship and u only have the height of the balloon and the angle of depression. So then the equation becomes :
The horizontal distance is given by the formula: tan θ=opposite/adjacent where: θ=21 opposite=2000 ft adjacent=a thus calculating for a we get: tan 21=2000/a thus a=2000/tan 21 a=5210.18 ft
