$$ \begin{aligned} 3(x+5) &= x+21&& \text{simplify left side} \\[1 em]3x+15 &= x+21&& \text{move the $ \color{blue}{ x } $ to the left side and $ \color{blue}{ 15 }$ to the right} \\[1 em]3x-x &= 21-15&& \text{simplify left and right hand side} \\[1 em]2x &= 6&& \text{ divide both sides by $ 2 $ } \\[1 em]x &= \frac{6}{2}&& \\[1 em]x &= 3&& \\[1 em] \end{aligned} $$

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