Answer
$|x^3y^3|\sqrt{y}$
Work Step by Step
Factor the radicand (expression inside the radical sign) so that at least one factor is a square to obtain:
$=\sqrt{x^6y^6(y)}
\\=\sqrt{(x^3y^3)^2(y)}$
The principal square root is always non-negative.
Thus, to make sure that the principal square root of $(x^3y^3)^2$ is non-negative, use absolute value bars to obtain:
$=|x^3y^3|\sqrt{y}$
