To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $7z$ and $6z$ is $42z$. Multiply $\frac{1}{7z}$ times $\frac{6}{6}$. Multiply $\frac{5}{6z}$ times $\frac{7}{7}$.
$$\frac{6}{42z}+\frac{5\times 7}{42z}$$
Since $\frac{6}{42z}$ and $\frac{5\times 7}{42z}$ have the same denominator, add them by adding their numerators.
