Answer
$-3xz^{3}$
Work Step by Step
$\sqrt[5] (-243x^{5}z^{15})=-3xz^{3}$, because $(-3xz^{3})^{5}=-3xz^{3}\times-3xz^{3}\times-3xz^{3}\times-3xz^{3}\times-3xz^{3}=$
$(-3\times-3\times-3\times-3\times-3)\times x^{1+1+1+1+1}\times z^{3+3+3+3+3}=-243x^{5}z^{15}$
