To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $6$ and $3$ is $6$. Multiply $\frac{1}{3}$ times $\frac{2}{2}$.
$$\frac{x}{6}-\frac{2}{6}$$
Since $\frac{x}{6}$ and $\frac{2}{6}$ have the same denominator, subtract them by subtracting their numerators.
