Multiply $ \dfrac{u}{7} $ by $ u $ to get $ \dfrac{ u^2 }{ 7 } $.

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

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{u}{7} \cdot u & \xlongequal{\text{Step 1}} \frac{u}{7} \cdot \frac{u}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ u \cdot u }{ 7 \cdot 1 } \xlongequal{\text{Step 3}} \frac{ u^2 }{ 7 } \end{aligned} $$

Multiply $ \dfrac{u^2}{7} $ by $ 2 $ to get $ \dfrac{ 2u^2 }{ 7 } $.

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

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{u^2}{7} \cdot 2 & \xlongequal{\text{Step 1}} \frac{u^2}{7} \cdot \frac{2}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ u^2 \cdot 2 }{ 7 \cdot 1 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 2u^2 }{ 7 } \end{aligned} $$

Add $ \dfrac{2u^2}{7} $ and $ 3u $ to get $ \dfrac{ \color{purple}{ 2u^2+21u } }{ 7 }$.

Step 1: Write $ 3u $ 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 second fraction by $\color{blue}{7}$.

$$ \begin{aligned} \frac{2u^2}{7} +3u & \xlongequal{\text{Step 1}} \frac{2u^2}{7} + \frac{3u}{\color{red}{1}} = \frac{ 2u^2 }{ 7 } + \frac{ 3u \cdot \color{blue}{ 7 }}{ 1 \cdot \color{blue}{ 7 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 2u^2 } }{ 7 } + \frac{ \color{purple}{ 21u } }{ 7 }=\frac{ \color{purple}{ 2u^2+21u } }{ 7 } \end{aligned} $$