In the equation given below, $K$ is a constant, if $x=6$, what is the value of $K$ ? <br> $\qquad \qquad \sqrt{K^2+9} -x =0$

Question

$\begin{array}{1 1} (a)\;3\sqrt{3} \\ (b)\; 2\sqrt{3} \\ (c)\; 3 \\ (d)\; 9 \end{array}$