To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $21$ and $7$ is $21$. Multiply $\frac{x}{7}$ times $\frac{3}{3}$.
$$\frac{1}{21}+\frac{3x}{21}$$
Since $\frac{1}{21}$ and $\frac{3x}{21}$ have the same denominator, add them by adding their numerators.
