Answer
$36y^{2}-9$
Work Step by Step
From Page 301, we know that the product of a sum and difference of two terms $x$ and $y$ is given by the following equation: $(x+y)(x-y)=x^{2}-y^{2}$.
Therefore, $(6y-3)(6y+3)=(6y)^{2}-3^{2}=6^{2}y^{2}-9=36y^{2}-9$.