To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $c$ and $8$ is $8c$. Multiply $\frac{3}{c}$ times $\frac{8}{8}$. Multiply $\frac{d}{8}$ times $\frac{c}{c}$.
$$\frac{3\times 8}{8c}+\frac{dc}{8c}$$
Since $\frac{3\times 8}{8c}$ and $\frac{dc}{8c}$ have the same denominator, add them by adding their numerators.
