Multiply $2x^3-14x^2$ by $ \dfrac{-4x+28}{7} $ to get $ \dfrac{-8x^4+112x^3-392x^2}{7} $.

Step 1: Write $ 2x^3-14x^2 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 2x^3-14x^2 \cdot \frac{-4x+28}{7} & \xlongequal{\text{Step 1}} \frac{2x^3-14x^2}{\color{red}{1}} \cdot \frac{-4x+28}{7} \xlongequal{\text{Step 2}} \frac{ \left( 2x^3-14x^2 \right) \cdot \left( -4x+28 \right) }{ 1 \cdot 7 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ -8x^4+56x^3+56x^3-392x^2 }{ 7 } = \frac{-8x^4+112x^3-392x^2}{7} \end{aligned} $$