Chapter 7 - Polynomial Equations and Factoring - 7.4 - Solving Polynomial Equations in Factored Form - Exercises - Page 381: 27

$3y^2(y-3)$.

The given expression is $=3y^3-9y^2$ The GCF of $3$ and $9$ is $3$. The GCF of $y^3$ and $y^2$ is $y^2$. So, the greatest common monomial factor of the terms is $3y^2$. Hence, the factor form is $3y^2(y-3)$.