$$ \begin{aligned} -56 &= 7\cdot(3+x)&& \text{simplify right side} \\[1 em]-56 &= 21+7x&& \\[1 em]-56 &= 7x+21&& \text{ swap the left and right side of the equation } \\[1 em]7x+21 &= -56&& \text{ move 21 to the right } \\[1 em]7x &= -56-21&& \\[1 em]7x &= -77&& \text{ divide both sides by $ 7 $ } \\[1 em]x &= -\frac{77}{7}&& \\[1 em]x &= -11&& \\[1 em] \end{aligned} $$

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