$$ \begin{aligned} \frac{3}{x} &= \frac{12}{52}&& \text{multiply ALL terms by } \color{blue}{ x\cdot52 }. \\[1 em]x\cdot52\cdot\frac{3}{x} &= x\cdot52\cdot\frac{12}{52}&& \text{cancel out the denominators} \\[1 em]156 &= 12x&& \text{ swap the left and right side of the equation } \\[1 em]12x &= 156&& \text{ divide both sides by $ 12 $ } \\[1 em]x &= \frac{156}{12}&& \\[1 em]x &= 13&& \\[1 em] \end{aligned} $$

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