Answer
$k=0.7$
Work Step by Step
Since $y$ varies directly as the square root of $x$, then, $
y=k\sqrt{x}
.$ Substituting the given values of the variables, then the value of $k$ is,
\begin{array}{l}
2.1=k\sqrt{9}
\\\\
2.1=k(3)
\\\\
\dfrac{2.1}{3}=k
\\\\
0.7=k
\\\\
k=0.7
.\end{array}
