To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $2$ and $3$ is $6$. Multiply $\frac{a}{2}$ times $\frac{3}{3}$. Multiply $\frac{5}{3}$ times $\frac{2}{2}$.
$$\frac{3a}{6}-\frac{5\times 2}{6}$$
Since $\frac{3a}{6}$ and $\frac{5\times 2}{6}$ have the same denominator, subtract them by subtracting their numerators.
