Answer
$x$ = $3\sqrt 3$
Work Step by Step
Using Pythagorean theorem in given Figure 1-
$x^{2} + 3^{2} $ = $6^{2}$
Or $x^{2} + 9 $ = $36$
Or $x^{2}$ = $36 - 9$ = 27
Therefore
$x$ = $\sqrt {27}$ = $\sqrt {9\times3}$ = $\sqrt {9} \sqrt 3$
$x$ = $3\sqrt 3$
