$$ \begin{aligned} 0 &= -\frac{2}{3}x+\frac{21}{3}&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3\cdot0 &= -3 \cdot \frac{2}{3}x+3\cdot\frac{21}{3}&& \text{cancel out the denominators} \\[1 em]0 &= -2x+21&& \text{ swap the left and right side of the equation } \\[1 em]-2x+21 &= 0&& \text{ move the constants to the right } \\[1 em]-2x &= -21&& \text{ divide both sides by $ -2 $ } \\[1 em]x &= \frac{-21}{-2}&& \\[1 em]x &= \frac{21}{2}&& \\[1 em] \end{aligned} $$

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