$$ \begin{aligned} -\frac{5}{2}(3x+4)-3x &= 0&& \text{multiply ALL terms by } \color{blue}{ 2 }. \\[1 em]-2 \cdot \frac{5}{2}(3x+4)-2\cdot3x &= 2\cdot0&& \text{cancel out the denominators} \\[1 em]-(15x+20)-6x &= 0&& \text{simplify left side} \\[1 em]-15x-20-6x &= 0&& \\[1 em]-21x-20 &= 0&& \text{ move the constants to the right } \\[1 em]-21x &= 20&& \text{ divide both sides by $ -21 $ } \\[1 em]x &= -\frac{20}{21}&& \\[1 em] \end{aligned} $$

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