#### Answer

The constant of variation is $0.7$.
The variation equation is $y=0.7\sqrt x$.

#### Work Step by Step

$y=k\sqrt x$
$y=2.1; x=9$
$2.1=k\sqrt 9$
$2.1=k(3)$
$k=0.7$
$y=0.7\sqrt x$
The constant of variation is $0.7$.
The variation equation is $y=0.7\sqrt x$.