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.