Multiply $ \dfrac{2}{3} $ by $ x $ to get $ \dfrac{ 2x }{ 3 } $.

Step 1: Write $ x $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{2}{3} \cdot x & \xlongequal{\text{Step 1}} \frac{2}{3} \cdot \frac{x}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ 2 \cdot x }{ 3 \cdot 1 } \xlongequal{\text{Step 3}} \frac{ 2x }{ 3 } \end{aligned} $$

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

Step 1: Write $ -x^3 $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: To add raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $\color{blue}{ 3 }$.

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