from 4 to 5,the difference is 1 ,and when you multiply by 3, then add 3 to 5 ,it gives you 8 ,to get 17,I multiple 3by 3 and add the answer to 8 ,and the same continues upto when you get the final answer which is 81.
difference is just multiplying the previous difference with 3 and add it with the previous number .
