$$ \begin{aligned} -56 &= -\frac{u}{6}&& \text{multiply ALL terms by } \color{blue}{ 6 }. \\[1 em]6\cdot(-56) &= -6 \cdot \frac{u}{6}&& \text{cancel out the denominators} \\[1 em]-336 &= -u&& \text{ swap the left and right side of the equation } \\[1 em]-u &= -336&& \text{switch signs on both sides} \\[1 em]u &= 336&& \\[1 em] \end{aligned} $$

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