Answer
(a) $6$
(b) $3x^2$
Work Step by Step
(a) Using property 2 (page 19):
$\sqrt {12}\sqrt 3=\sqrt {12\times3}=\sqrt {36}=6$
(b) Using properties 6 (page 19):
$\sqrt[4] {(3x^2)^4}=|3x^2|=3x^2$
Obs: $3x^2\geq0$ for any value of $x$, so: $|3x^2|=3x^2$
