Draw a right triangle from the plane (point A) parallel to the ground to a point right above the airport (point C) and from point C down to the airport (point B) and then from point B back to point A.
 There is a right angle at point C.
 AC = 4 miles 
 angle A is 6.5 degrees (the angle of elevation is the same as the angle of depression)
 In right triangle ACB, you know angle A and its adjacent side, AC. You need to know its opposite side CB.
 tan = opp/adj    --->    tan 6.5  =  x / 4   --->  x = 4 · tan(6.5)
 Solve for x.