$$ \begin{aligned} -\frac{40}{40}-\frac{x}{49} &= -\frac{66}{49}&& \text{multiply ALL terms by } \color{blue}{ 1960 }. \\[1 em]-1960\cdot\frac{40}{40}-1960\frac{x}{49} &= -1960\cdot\frac{66}{49}&& \text{cancel out the denominators} \\[1 em]-1960-40x &= -2640&& \text{ move -1960 to the right } \\[1 em]-40x &= -2640+1960&& \\[1 em]-40x &= -680&& \text{ divide both sides by $ -40 $ } \\[1 em]x &= \frac{-680}{-40}&& \\[1 em]x &= 17&& \\[1 em] \end{aligned} $$

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