Multiply $12$ by $ \dfrac{43}{b} $ to get $ \dfrac{ 516 }{ b } $.

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

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 12 \cdot \frac{43}{b} & \xlongequal{\text{Step 1}} \frac{12}{\color{red}{1}} \cdot \frac{43}{b} \xlongequal{\text{Step 2}} \frac{ 12 \cdot 43 }{ 1 \cdot b } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 516 }{ b } \end{aligned} $$

Subtract $ \dfrac{516}{b} $ from $ \dfrac{23}{b} $ to get $ \dfrac{-493}{b} $.

To subtract expressions with the same denominators, we subtract the numerators and write the result over the common denominator.

$$ \begin{aligned} \frac{23}{b} - \frac{516}{b} & = \frac{23}{\color{blue}{b}} - \frac{516}{\color{blue}{b}} =\frac{ 23 - 516 }{ \color{blue}{ b }} = \\[1ex] &= \frac{-493}{b} \end{aligned} $$