$$ \begin{aligned} 6 &= -\frac{4}{3}u&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3\cdot6 &= -3 \cdot \frac{4}{3}u&& \text{cancel out the denominators} \\[1 em]18 &= -4u&& \text{ swap the left and right side of the equation } \\[1 em]-4u &= 18&& \text{ divide both sides by $ -4 $ } \\[1 em]u &= -\frac{18}{4}&& \\[1 em]u &= -\frac{9}{2}&& \\[1 em] \end{aligned} $$

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