Consider $36a^{2}-25b^{2}$. Rewrite $36a^{2}-25b^{2}$ as $\left(6a\right)^{2}-\left(5b\right)^{2}$. The difference of squares can be factored using the rule: $p^{2}-q^{2}=\left(p-q\right)\left(p+q\right)$.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $25$ and $36$ is $900$. Multiply $\frac{a^{2}}{25}$ times $\frac{36}{36}$. Multiply $\frac{b^{2}}{36}$ times $\frac{25}{25}$.
$$\frac{36a^{2}}{900}-\frac{25b^{2}}{900}$$
Since $\frac{36a^{2}}{900}$ and $\frac{25b^{2}}{900}$ have the same denominator, subtract them by subtracting their numerators.
