(iii) Use your answer to part (c)(ii), and the method of completing the square, to determine the distance, to the…

(iii) Use your answer to part (c)(ii), and the method of completing the square, to determine the distance, to the nearest kilometre, between the aeroplane and the aircraft detection tower at the point on the line of flight of the aeroplane where it is closest to the aircraft detection tower. (The distance required is the ‘horizontal’ distance; that is, the distance between the aircraft detection tower and the point on the ground immediately below the aeroplane.)