f √6 is the geometric mean between 6 and another number, then the number is

6*x = √6
What you have here is a geometric series beginning with x ⇒ √6 ⇒ 6
x = √6 / 6 It is the smallest number in the series.

Each number is multiplied by √6 to get to the next number in the series.
√6/6 <<<< answer