$$\frac{3}{7}+\frac{x}{3} = \frac{11}{14}$$

$$ \begin{aligned} \frac{3}{7}+\frac{x}{3} &= \frac{11}{14}&& \text{multiply ALL terms by } \color{blue}{ 42 }. \\[1 em]42\cdot\frac{3}{7}+42\frac{x}{3} &= 42\cdot\frac{11}{14}&& \text{cancel out the denominators} \\[1 em]18+14x &= 33&& \text{ move 18 to the right } \\[1 em]14x &= 33-18&& \\[1 em]14x &= 15&& \text{ divide both sides by $ 14 $ } \\[1 em]x &= \frac{15}{14}&& \\[1 em] \end{aligned} $$

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