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