it is not related to Computer Networks... it's just uses the terminologies form CN.. it is a aptitude question.
coming to the question:-
let take initially there are x million users
2008-1996 = 12 years = 12 * 12 months ====> y =(12 * 12) / 18 = 8 steps required
there is doubling on 18 months ==> 2*x
on 36 ( 2*18 ) months, it would be ===> 2*2*x = 22 * x
on 54 ( 3 *18 ) months, it would be ===> 2*2*2*x = 23 * x
on 72 ( 4 * 18 ) months, it would be ===> 2*2*2*2*x = 24 * x
on 12*12 ( 8 * 18 ) months, it would be ===> 2*2*2*2*x = 28 * x
answer wants in billions ===> 28 * x millions/1000 = ( 256 * 7) /1000 = 1.792