To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $y+3$ and $y^{2}+9$ is $\left(y+3\right)\left(y^{2}+9\right)$. Multiply $\frac{1}{y+3}$ times $\frac{y^{2}+9}{y^{2}+9}$. Multiply $\frac{6}{y^{2}+9}$ times $\frac{y+3}{y+3}$.
Since $\frac{y^{2}+9}{\left(y+3\right)\left(y^{2}+9\right)}$ and $\frac{6\left(y+3\right)}{\left(y+3\right)\left(y^{2}+9\right)}$ have the same denominator, add them by adding their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $\left(y+3\right)\left(y^{2}+9\right)$ and $12y^{2}$ is $12\left(y+3\right)y^{2}\left(y^{2}+9\right)$. Multiply $\frac{y^{2}+27+6y}{\left(y+3\right)\left(y^{2}+9\right)}$ times $\frac{12y^{2}}{12y^{2}}$. Multiply $\frac{1}{12y^{2}}$ times $\frac{\left(y+3\right)\left(y^{2}+9\right)}{\left(y+3\right)\left(y^{2}+9\right)}$.
Since $\frac{\left(y^{2}+27+6y\right)\times 12y^{2}}{12\left(y+3\right)y^{2}\left(y^{2}+9\right)}$ and $\frac{\left(y+3\right)\left(y^{2}+9\right)}{12\left(y+3\right)y^{2}\left(y^{2}+9\right)}$ have the same denominator, subtract them by subtracting their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $y+3$ and $y^{2}+9$ is $\left(y+3\right)\left(y^{2}+9\right)$. Multiply $\frac{1}{y+3}$ times $\frac{y^{2}+9}{y^{2}+9}$. Multiply $\frac{6}{y^{2}+9}$ times $\frac{y+3}{y+3}$.
Since $\frac{y^{2}+9}{\left(y+3\right)\left(y^{2}+9\right)}$ and $\frac{6\left(y+3\right)}{\left(y+3\right)\left(y^{2}+9\right)}$ have the same denominator, add them by adding their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $\left(y+3\right)\left(y^{2}+9\right)$ and $12y^{2}$ is $12\left(y+3\right)y^{2}\left(y^{2}+9\right)$. Multiply $\frac{y^{2}+27+6y}{\left(y+3\right)\left(y^{2}+9\right)}$ times $\frac{12y^{2}}{12y^{2}}$. Multiply $\frac{1}{12y^{2}}$ times $\frac{\left(y+3\right)\left(y^{2}+9\right)}{\left(y+3\right)\left(y^{2}+9\right)}$.
Since $\frac{\left(y^{2}+27+6y\right)\times 12y^{2}}{12\left(y+3\right)y^{2}\left(y^{2}+9\right)}$ and $\frac{\left(y+3\right)\left(y^{2}+9\right)}{12\left(y+3\right)y^{2}\left(y^{2}+9\right)}$ have the same denominator, subtract them by subtracting their numerators.