Answer
$a_{n}=(2n)!$
Work Step by Step
We recognize factorials.
$1!=1$
$2!=2\quad*$
$3!=6$
$4!=24\quad*$
$5!=120$
$6!=720\quad*$
$...$
1st term: 2=2!
2nd term: 24=4!
third term: 720=6!
fourth term: 40,320=8!
...
n-th term: (2n)!
$a_{n}=(2n)!$
