Answer
$21s^{6}t^{11}$
Work Step by Step
Based on the product rule for exponents, we know that $a^{m}\times a^{n}=a^{m+n}$ (where $m$ and $n$ are positive integers and $a$ is a real number).
Therefore, $(-3s^{5}t)(-7st^{10})=-3\times-7\times s^{5+1}\times t^{1+10}=21s^{6}t^{11}$.