#### Answer

$\sqrt{x^2-36}$

#### Work Step by Step

RECALL:
(i) For nonegative numbers a and b,
$\sqrt[n]{a} \cdot \sqrt[n]{b} = \sqrt[n]{ab}$
(ii) $(a+b)(a-b)=a^{2}-b^2$
Use the rules above to obtain:
$=\sqrt{(x+6)(x-6)}
\\=\sqrt{x^{2}-6^{2}}
\\=\sqrt{x^2-36}$