To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $2$ and $3$ is $6$. Multiply $\frac{1a}{2}$ times $\frac{3}{3}$. Multiply $\frac{2b}{3}$ times $\frac{2}{2}$.
$$\frac{3\times 1a}{6}+\frac{2\times 2b}{6}$$
Since $\frac{3\times 1a}{6}$ and $\frac{2\times 2b}{6}$ have the same denominator, add them by adding their numerators.
$$\frac{3\times 1a+2\times 2b}{6}$$
Do the multiplications in $3\times 1a+2\times 2b$.
