Answer
$\sqrt{x^3} \text{ or } \left(\sqrt{x}\right)^3$
Work Step by Step
Note that:
$x^{1.5}=x^{\frac{3}{2}}$
Recall the rational exponent property (pg. 382):
$a^{\frac{m}{n}}=\sqrt[n]{a^m}=(\sqrt[n]{a})^m$
Applying this property ($m=3$, $n=2$, $a=x$), we get:
$\sqrt{x^3}=\left(\sqrt{x}\right)^3$