$$ \begin{aligned} 4(x+9) &= 9(x+4)&& \text{simplify left and right hand side} \\[1 em]4x+36 &= 9x+36&& \text{move the $ \color{blue}{ 9x } $ to the left side and $ \color{blue}{ 36 }$ to the right} \\[1 em]4x-9x &= 36-36&& \text{simplify left and right hand side} \\[1 em]-5x &= 0&& \text{ divide both sides by $ -5 $ } \\[1 em]x &= \frac{0}{-5}&& \\[1 em]x &= 0&& \\[1 em] \end{aligned} $$

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