Add $ \dfrac{4}{3x-3} $ and $ \dfrac{x+2}{x^2-x} $ to get $ \dfrac{ \color{purple}{ 7x+6 } }{ 3x^2-3x }$.

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}{ x }$ and the second by $\color{blue}{ 3 }$.

$$ \begin{aligned} \frac{4}{3x-3} + \frac{x+2}{x^2-x} & = \frac{ 4 \cdot \color{blue}{ x }}{ \left( 3x-3 \right) \cdot \color{blue}{ x }} + \frac{ \left( x+2 \right) \cdot \color{blue}{ 3 }}{ \left( x^2-x \right) \cdot \color{blue}{ 3 }} = \\[1ex] &=\frac{ \color{purple}{ 4x } }{ 3x^2-3x } + \frac{ \color{purple}{ 3x+6 } }{ 3x^2-3x }=\frac{ \color{purple}{ 7x+6 } }{ 3x^2-3x } \end{aligned} $$