Multiply $\frac{3}{5}$ times $\frac{1}{b}$ by multiplying numerator times numerator and denominator times denominator.
$$\frac{21}{10}-\frac{3}{5b}$$
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $10$ and $5b$ is $10b$. Multiply $\frac{21}{10}$ times $\frac{b}{b}$. Multiply $\frac{3}{5b}$ times $\frac{2}{2}$.
$$\frac{21b}{10b}-\frac{3\times 2}{10b}$$
Since $\frac{21b}{10b}$ and $\frac{3\times 2}{10b}$ have the same denominator, subtract them by subtracting their numerators.
