To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $6$ and $7$ is $42$. Multiply $\frac{x}{6}$ times $\frac{7}{7}$. Multiply $\frac{2x}{7}$ times $\frac{6}{6}$.
$$\frac{7x}{42}-\frac{6\times 2x}{42}$$
Since $\frac{7x}{42}$ and $\frac{6\times 2x}{42}$ have the same denominator, subtract them by subtracting their numerators.
