Answer
$-15y^{5}$
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, $(3y^{4})(-5y)=3\times-5\times y^{4+1}=-15y^{5}$.