To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $3a^{2}$ and $4a$ is $12a^{2}$. Multiply $\frac{a-1}{3a^{2}}$ times $\frac{4}{4}$. Multiply $\frac{a+3}{4a}$ times $\frac{3a}{3a}$.
Since $\frac{4\left(a-1\right)}{12a^{2}}$ and $\frac{\left(a+3\right)\times 3a}{12a^{2}}$ have the same denominator, add them by adding their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $3a^{2}$ and $4a$ is $12a^{2}$. Multiply $\frac{a-1}{3a^{2}}$ times $\frac{4}{4}$. Multiply $\frac{a+3}{4a}$ times $\frac{3a}{3a}$.
Since $\frac{4\left(a-1\right)}{12a^{2}}$ and $\frac{\left(a+3\right)\times 3a}{12a^{2}}$ have the same denominator, add them by adding their numerators.