For a geometric sequence
a, ar, ar ², ar ³, …
the n-th term in the sequence is ar ⁿ ⁻ ¹.
The first sequence is
1, 3, 9, 27, …
so it's clear that a = 1 and r = 3, and so the n-th term is 3ⁿ ⁻ ¹.
The second sequence is
400, 200, 100, 50, …
so of course a = 400, and you can easily solve for r :
200 = 400r ==> r = 200/400 = 1/2
Then the n-th term is 400 (1/2)ⁿ ⁻ ¹.
Similarly, the other sequences are given by
3rd: … 4 × 2ⁿ ⁻ ¹
4th: … 400 (1/4)ⁿ ⁻ ¹
5th: … 5ⁿ ⁻ ¹
6th: … 1000 (1/2)ⁿ ⁻ ¹
7th: … 2 × 5ⁿ ⁻ ¹