The sum of the squared legs must give the squared hypotenuse:
[tex](\sqrt{6})^2+x^2=6^2 \iff x^2+6=36 \iff x^2=30\iff x=\pm\sqrt{30}[/tex]
But of course we can only accept the positive solution, because [tex]x[/tex] is a length and can't be negative.
