To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $\left(x-4\right)\left(x+4\right)$ and $2x\left(x+4\right)$ is $2x\left(x-4\right)\left(x+4\right)$. Multiply $\frac{x^{2}+12x+36}{\left(x-4\right)\left(x+4\right)}$ times $\frac{2x}{2x}$. Multiply $\frac{3x+18}{2x\left(x+4\right)}$ times $\frac{x-4}{x-4}$.
Since $\frac{\left(x^{2}+12x+36\right)\times 2x}{2x\left(x-4\right)\left(x+4\right)}$ and $\frac{\left(3x+18\right)\left(x-4\right)}{2x\left(x-4\right)\left(x+4\right)}$ 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 $\left(x-4\right)\left(x+4\right)$ and $2x\left(x+4\right)$ is $2x\left(x-4\right)\left(x+4\right)$. Multiply $\frac{x^{2}+12x+36}{\left(x-4\right)\left(x+4\right)}$ times $\frac{2x}{2x}$. Multiply $\frac{3x+18}{2x\left(x+4\right)}$ times $\frac{x-4}{x-4}$.
Since $\frac{\left(x^{2}+12x+36\right)\times 2x}{2x\left(x-4\right)\left(x+4\right)}$ and $\frac{\left(3x+18\right)\left(x-4\right)}{2x\left(x-4\right)\left(x+4\right)}$ have the same denominator, add them by adding their numerators.