After 1st week, subscribers = 0
After 2nd week, subscribers = 0
After 3rd week, subscribers = 1
After 4th week, subscribers = 1x4 = 4
After 5th week, subscribers = 4x4 = 4^2 = 4^(5-3)
After 6th week, subscribers = 4x4x4= 4^3=4^(6-3)
.
.
.
Therefore, after 12th week, subscribers =
4^(12-3) = 4^9 = 262144