If the plane is flying at an altitude of 21,000 feet, it is perpendicular to the observer's horizontal line of sight. Thus, I concluded that the given information be assigned to the attachment below.
Given -
( Observer at point O, plane flying at point P
( Plane at 55° angle to horizontal line of sight, altitude of 21,000 feet
The distance ( x ), is represented by the line segment OP, with which we have to determine the length of. Therefore we can conclude the following -

As you can see, the distance from the plane to the observer is about 25,636 ft!