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