To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $y^{2}-2y+4$ and $y^{2}+2y+4$ is $\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)$. Multiply $\frac{y-2}{y^{2}-2y+4}$ times $\frac{y^{2}+2y+4}{y^{2}+2y+4}$. Multiply $\frac{y+2}{y^{2}+2y+4}$ times $\frac{y^{2}-2y+4}{y^{2}-2y+4}$.
Since $\frac{\left(y-2\right)\left(y^{2}+2y+4\right)}{\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)}$ and $\frac{\left(y+2\right)\left(y^{2}-2y+4\right)}{\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)}$ have the same denominator, add them by adding their numerators.
Since $\frac{2y^{3}}{\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)}$ and $\frac{16}{\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)}$ 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 $y^{2}-2y+4$ and $y^{2}+2y+4$ is $\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)$. Multiply $\frac{y-2}{y^{2}-2y+4}$ times $\frac{y^{2}+2y+4}{y^{2}+2y+4}$. Multiply $\frac{y+2}{y^{2}+2y+4}$ times $\frac{y^{2}-2y+4}{y^{2}-2y+4}$.
Since $\frac{\left(y-2\right)\left(y^{2}+2y+4\right)}{\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)}$ and $\frac{\left(y+2\right)\left(y^{2}-2y+4\right)}{\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)}$ have the same denominator, add them by adding their numerators.
Since $\frac{2y^{3}}{\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)}$ and $\frac{16}{\left(y^{2}-2y+4\right)\left(y^{2}+2y+4\right)}$ have the same denominator, subtract them by subtracting their numerators.