Answer
$$\frac{3\left(y-3\right)}{2\left(y+3\right)}$$
Work Step by Step
Recall, to simplify rational expressions, we look for common factors in the numerator and the denominator. Doing this, we find:
$$ \frac{6y^2-36y+54}{4y^2-36}\\ \frac{3\left(y-3\right)^2}{2\left(y+3\right)\left(y-3\right)}\\ \frac{3\left(y-3\right)}{2\left(y+3\right)}$$