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