An important and exam oriented question as it is already asked in previous year paper of 2013 from trigonometry(height and distance) in which we are to find the height of the building if the angle of elevation of the top of the building from the foot of the tower is 30 and the angle of elevation of the top of the tower from the foot of the building is 60.
Book RS Aggarwal, Class 10, chapter 14, question no 16
Hope it helps you