Answer
$2|x^3|\sqrt{6}$
Work Step by Step
RECALL:
$\sqrt{a} \cdot \sqrt{b} = \sqrt{a\cdot b}, a\gt0, b\gt 0$.
Use the rule above to have
$\sqrt{(8x^5)(3x)}
\\=\sqrt{24x^6}
\\=\sqrt{4x^6(6)}
\\=\sqrt{(2x^3)^2(6)}
\\=2|x^3|\sqrt{6}.$