Multiply $ \dfrac{n}{10} $ by $ n^2 $ to get $ \dfrac{ n^3 }{ 10 } $.

Step 1: Write $ n^2 $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{n}{10} \cdot n^2 & \xlongequal{\text{Step 1}} \frac{n}{10} \cdot \frac{n^2}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ n \cdot n^2 }{ 10 \cdot 1 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ n^3 }{ 10 } \end{aligned} $$

Subtract $3n$ from $ \dfrac{n^3}{10} $ to get $ \dfrac{ \color{purple}{ n^3-30n } }{ 10 }$.

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

Step 2: To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the second fraction by $\color{blue}{10}$.

$$ \begin{aligned} \frac{n^3}{10} -3n & \xlongequal{\text{Step 1}} \frac{n^3}{10} - \frac{3n}{\color{red}{1}} = \frac{ n^3 }{ 10 } - \frac{ 3n \cdot \color{blue}{ 10 }}{ 1 \cdot \color{blue}{ 10 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ n^3 } }{ 10 } - \frac{ \color{purple}{ 30n } }{ 10 }=\frac{ \color{purple}{ n^3-30n } }{ 10 } \end{aligned} $$