# Next number in this sequence?

I was trying this 3,9,18,______,648 What is the missing number? Any hints?

I am given options as:

1. 27
2. 324
3. 54
4. 108

I thought of adding the digits and then see a pattern like except 3 all the digits of the numbers add to 9, but that is for all the numbers given in the options.

$3^2=9$
$9\cdot2=18$
$18^2=\boxed{324}$
$324\cdot2=648$