Question
$$7+9d = 7d+3$$
Answer
$$ d = -2 $$
Explanation
$$ \begin{aligned} 7+9d &= 7d+3&& \text{move the $ \color{blue}{ 7d } $ to the left side and $ \color{blue}{ 7 }$ to the right} \\[1 em]9d-7d &= 3-7&& \text{simplify left and right hand side} \\[1 em]2d &= -4&& \text{ divide both sides by $ 2 $ } \\[1 em]d &= -\frac{4}{2}&& \\[1 em]d &= -2&& \\[1 em] \end{aligned} $$
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