$$18d-21 = 15d+3$$
Answer
$$ d = 8 $$
Explanation
$$ \begin{aligned} 18d-21 &= 15d+3&& \text{move the $ \color{blue}{ 15d } $ to the left side and $ \color{blue}{ -21 }$ to the right} \\[1 em]18d-15d &= 3+21&& \text{simplify left and right hand side} \\[1 em]3d &= 24&& \text{ divide both sides by $ 3 $ } \\[1 em]d &= \frac{24}{3}&& \\[1 em]d &= 8&& \\[1 em] \end{aligned} $$
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