$$ \begin{aligned} -21 &= \frac{1}{2}x-11&& \text{multiply ALL terms by } \color{blue}{ 2 }. \\[1 em]2\cdot(-21) &= 2 \cdot \frac{1}{2}x-2\cdot11&& \text{cancel out the denominators} \\[1 em]-42 &= x-22&& \text{ swap the left and right side of the equation } \\[1 em]x-22 &= -42&& \text{ move -22 to the right } \\[1 em]x &= -42+22&& \\[1 em]x &= -20&& \\[1 em]x &= -20&& \\[1 em] \end{aligned} $$

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