To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $6$ and $8$ is $24$. Multiply $\frac{s}{6}$ times $\frac{4}{4}$. Multiply $\frac{3}{8}$ times $\frac{3}{3}$.
$$\frac{4s}{24}-\frac{3\times 3}{24}$$
Since $\frac{4s}{24}$ and $\frac{3\times 3}{24}$ have the same denominator, subtract them by subtracting their numerators.
