$$ \begin{aligned} -18+15 &= 3(x+6)&& \text{simplify left and right hand side} \\[1 em]-3 &= 3x+18&& \text{ swap the left and right side of the equation } \\[1 em]3x+18 &= -3&& \text{ move 18 to the right } \\[1 em]3x &= -3-18&& \\[1 em]3x &= -21&& \text{ divide both sides by $ 3 $ } \\[1 em]x &= -\frac{21}{3}&& \\[1 em]x &= -7&& \\[1 em] \end{aligned} $$

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