To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $4$ and $9b^{2}$ is $36b^{2}$. Multiply $\frac{9b^{2}}{4}$ times $\frac{9b^{2}}{9b^{2}}$. Multiply $\frac{4}{9b^{2}}$ times $\frac{4}{4}$.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $24b$ and $54b^{3}$ is $216b^{3}$. Multiply $\frac{81b^{4}-36b^{2}+16}{24b}$ times $\frac{9b^{2}}{9b^{2}}$. Multiply $\frac{81b^{4}-36b^{2}+16}{54b^{3}}$ times $\frac{4}{4}$.
Since $\frac{\left(81b^{4}-36b^{2}+16\right)\times 9b^{2}}{216b^{3}}$ and $\frac{4\left(81b^{4}-36b^{2}+16\right)}{216b^{3}}$ 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 $4$ and $9b^{2}$ is $36b^{2}$. Multiply $\frac{9b^{2}}{4}$ times $\frac{9b^{2}}{9b^{2}}$. Multiply $\frac{4}{9b^{2}}$ times $\frac{4}{4}$.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $24b$ and $54b^{3}$ is $216b^{3}$. Multiply $\frac{81b^{4}-36b^{2}+16}{24b}$ times $\frac{9b^{2}}{9b^{2}}$. Multiply $\frac{81b^{4}-36b^{2}+16}{54b^{3}}$ times $\frac{4}{4}$.
Since $\frac{\left(81b^{4}-36b^{2}+16\right)\times 9b^{2}}{216b^{3}}$ and $\frac{4\left(81b^{4}-36b^{2}+16\right)}{216b^{3}}$ have the same denominator, add them by adding their numerators.