Multiply $12a^5$ by $ \dfrac{b^4}{18} $ to get $ \dfrac{ 12a^5b^4 }{ 18 } $.

Step 1: Write $ 12a^5 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 12a^5 \cdot \frac{b^4}{18} & \xlongequal{\text{Step 1}} \frac{12a^5}{\color{red}{1}} \cdot \frac{b^4}{18} \xlongequal{\text{Step 2}} \frac{ 12a^5 \cdot b^4 }{ 1 \cdot 18 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 12a^5b^4 }{ 18 } \end{aligned} $$

Multiply $ \dfrac{12a^5b^4}{18} $ by $ a $ to get $ \dfrac{ 12a^6b^4 }{ 18 } $.

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

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{12a^5b^4}{18} \cdot a & \xlongequal{\text{Step 1}} \frac{12a^5b^4}{18} \cdot \frac{a}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ 12a^5b^4 \cdot a }{ 18 \cdot 1 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 12a^6b^4 }{ 18 } \end{aligned} $$

Multiply $ \dfrac{12a^6b^4}{18} $ by $ b^2 $ to get $ \dfrac{ 12a^6b^6 }{ 18 } $.

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

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{12a^6b^4}{18} \cdot b^2 & \xlongequal{\text{Step 1}} \frac{12a^6b^4}{18} \cdot \frac{b^2}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ 12a^6b^4 \cdot b^2 }{ 18 \cdot 1 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 12a^6b^6 }{ 18 } \end{aligned} $$