common ratio, r = 12/6 =2
nth term of geometric progression is given by, an = a1 * rn-1
In this question , an = 1536, a1 = 6 and r =2
therefore, 1536 = 6 * 2n-1
or 2n-1= 1536/6
or 2n-1= 256
or 2n-1= 28
or n-1 = 8
or n =9
So there are 9 terms in the given progression.