An object is located at x = 0. At x = 2.50 cm is a converging lens with a focal length of 2.00 cm, at x = 16.5 cm is an unknown lens, and at x = 19.8 cm is another converging lens with focal length 4.00 cm. An upright image is formed at x = 39.8 cm. For each lens, the object distance exceeds the focal length. The magnification of the system is 6.84. 

(a) Is the unknown lens diverging or converging? 

(b) What is the focal length of the unknown lens?