Finding the $n^{\text{th}}$ Term of a Sequence

Example 1:

In the sequence $2,6,18,54,\mathrm{...}$  the first term is
$a_1=2$ , the second term is $a_2=6$ and so forth.

A term is multiplied by $3$ to get the next term.

Example 2:

$a_n=n^2$

First term: $a_1=1^2=1$

Second term: $a_2=2^2=4$

Fifteenth term: $a_{15}={15}^2=225$