Answer
$r^{17}t^{11}$
Work Step by Step
Using $(ab)^x=a^xb^x$, then the expression, $
(r^5t)^{3}(r^2t^8)
$, simplifies to
\begin{array}{l}\require{cancel}
(r^{5(3)}t^{3})(r^2t^8)
\\\\=
(r^{15}t^{3})(r^2t^8)
.\end{array}
Using $a^x\cdot a^y=a^{x+y}$, then given expression, $
(r^{15}t^{3})(r^2t^8)
$, simplifies to
\begin{array}{l}\require{cancel}
r^{15+2}t^{3+8}
\\\\=
r^{17}t^{11}
.\end{array}
