Add $ \dfrac{3}{3a-3} $ and $ \dfrac{3a}{6} $ to get $ \dfrac{ \color{purple}{ 9a^2-9a+18 } }{ 18a-18 }$.

To add raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 6 }$ and the second by $\color{blue}{ 3a-3 }$.

$$ \begin{aligned} \frac{3}{3a-3} + \frac{3a}{6} & = \frac{ 3 \cdot \color{blue}{ 6 }}{ \left( 3a-3 \right) \cdot \color{blue}{ 6 }} + \frac{ 3a \cdot \color{blue}{ \left( 3a-3 \right) }}{ 6 \cdot \color{blue}{ \left( 3a-3 \right) }} = \\[1ex] &=\frac{ \color{purple}{ 18 } }{ 18a-18 } + \frac{ \color{purple}{ 9a^2-9a } }{ 18a-18 }=\frac{ \color{purple}{ 9a^2-9a+18 } }{ 18a-18 } \end{aligned} $$