Answer
$f(3\sqrt{5}).f(–3\sqrt{5}) = 6$
Work Step by Step
$f(3\sqrt{5}) = \sqrt{9+3\sqrt{5}}$
$f(–3\sqrt{5}) = \sqrt{9–3\sqrt{5}}$
Now
$f(3\sqrt{5}).f(–3\sqrt{5}) = \sqrt{9+3\sqrt{5}}\times\sqrt{9–3\sqrt{5}}$
$= \sqrt{(9+3\sqrt{5})\times(9–3\sqrt{5})} =\sqrt{9^2 –(3\sqrt{5})^2}$
$= \sqrt{81–45} = \sqrt{36} = 6$