$$ \begin{aligned} 2 \cdot \frac{x}{5} &= 9-\frac{x}{2}&& \text{multiply ALL terms by } \color{blue}{ 10 }. \\[1 em]10\cdot2 \cdot \frac{x}{5} &= 10\cdot9-10 \cdot \frac{x}{2}&& \text{cancel out the denominators} \\[1 em]4x &= 90-5x&& \text{move the $ \color{blue}{ -5x } $ to the left side} \\[1 em]4x+5x &= 90&& \text{simplify left side} \\[1 em]9x &= 90&& \text{ divide both sides by $ 9 $ } \\[1 em]x &= \frac{90}{9}&& \\[1 em]x &= 10&& \\[1 em] \end{aligned} $$

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