To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $x-3$ and $x-4$ is $\left(x-4\right)\left(x-3\right)$. Multiply $\frac{x-4}{x-3}$ times $\frac{x-4}{x-4}$. Multiply $\frac{x-5}{x-4}$ times $\frac{x-3}{x-3}$.
Since $\frac{\left(x-4\right)\left(x-4\right)}{\left(x-4\right)\left(x-3\right)}$ and $\frac{\left(x-5\right)\left(x-3\right)}{\left(x-4\right)\left(x-3\right)}$ 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 $\left(x-4\right)\left(x-3\right)$ and $x-5$ is $\left(x-5\right)\left(x-4\right)\left(x-3\right)$. Multiply $\frac{2x^{2}-16x+31}{\left(x-4\right)\left(x-3\right)}$ times $\frac{x-5}{x-5}$. Multiply $\frac{x-3}{x-5}$ times $\frac{\left(x-4\right)\left(x-3\right)}{\left(x-4\right)\left(x-3\right)}$.
Since $\frac{\left(2x^{2}-16x+31\right)\left(x-5\right)}{\left(x-5\right)\left(x-4\right)\left(x-3\right)}$ and $\frac{\left(x-3\right)\left(x-4\right)\left(x-3\right)}{\left(x-5\right)\left(x-4\right)\left(x-3\right)}$ 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 $x-3$ and $x-4$ is $\left(x-4\right)\left(x-3\right)$. Multiply $\frac{x-4}{x-3}$ times $\frac{x-4}{x-4}$. Multiply $\frac{x-5}{x-4}$ times $\frac{x-3}{x-3}$.
Since $\frac{\left(x-4\right)\left(x-4\right)}{\left(x-4\right)\left(x-3\right)}$ and $\frac{\left(x-5\right)\left(x-3\right)}{\left(x-4\right)\left(x-3\right)}$ 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 $\left(x-4\right)\left(x-3\right)$ and $x-5$ is $\left(x-5\right)\left(x-4\right)\left(x-3\right)$. Multiply $\frac{2x^{2}-16x+31}{\left(x-4\right)\left(x-3\right)}$ times $\frac{x-5}{x-5}$. Multiply $\frac{x-3}{x-5}$ times $\frac{\left(x-4\right)\left(x-3\right)}{\left(x-4\right)\left(x-3\right)}$.
Since $\frac{\left(2x^{2}-16x+31\right)\left(x-5\right)}{\left(x-5\right)\left(x-4\right)\left(x-3\right)}$ and $\frac{\left(x-3\right)\left(x-4\right)\left(x-3\right)}{\left(x-5\right)\left(x-4\right)\left(x-3\right)}$ have the same denominator, add them by adding their numerators.