Answer
$3a\sqrt{{3ab}}$
Work Step by Step
RECALL:
For any non-negative real number,
$\sqrt{a^2} = a$
Factor the radicand (which is the expression inside the radical sign) so that at least one factor is a square, and then simplify using the rule above to obtain:
$=\sqrt{9a^2(3ab)}
\\=\sqrt{(3a)^2(3ab)}
\\=3a\sqrt{{3ab}}$
