Answer
$27y^{5}$
Work Step by Step
We are given the expression $-3y\times-9y^{4}$. We can simplify by grouping like terms.
$(-3\times-9)\times(y\times y^{4})$
Next, we can use the product rule, which holds that $a^{m}\times{a^{n}}=a^{m+n}$ (where a is a real number, and m and n are positive integers).
$27\times(y^{1+4})=27y^{5}$