$$ \begin{aligned} 11-\frac{2}{a}+\frac{21}{a} &= 0&& \text{multiply ALL terms by } \color{blue}{ a }. \\[1 em]a\cdot11-a\cdot\frac{2}{a}+a\cdot\frac{21}{a} &= a\cdot0&& \text{cancel out the denominators} \\[1 em]11a-2+21 &= 0&& \text{simplify left side} \\[1 em]11a+19 &= 0&& \text{ move the constants to the right } \\[1 em]11a &= -19&& \text{ divide both sides by $ 11 $ } \\[1 em]a &= -\frac{19}{11}&& \\[1 em] \end{aligned} $$

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