$$ \begin{aligned} 42 &= 42+9b&& \text{ swap the left and right side of the equation } \\[1 em]42+9b &= 42&& \text{ move 42 to the right } \\[1 em]9b &= 42-42&& \\[1 em]9b &= 0&& \text{ divide both sides by $ 9 $ } \\[1 em]b &= \frac{0}{9}&& \\[1 em]b &= 0&& \\[1 em] \end{aligned} $$