To find the geometric mean,
Simply follow the steps below:
[tex]\frac{x}{7\text{ }}=\frac{12}{x}[/tex][tex]x^2=84[/tex]Take the square root of both-side
[tex]x=\sqrt[]{84}[/tex][tex]=\sqrt[]{4\text{ x 21}}[/tex][tex]=2\sqrt[]{21}[/tex]
Therefore, the correct option is C.2√21
