$$48 = 45-\frac{y}{6}$$

$$ \begin{aligned} 48 &= 45-\frac{y}{6}&& \text{multiply ALL terms by } \color{blue}{ 6 }. \\[1 em]6\cdot48 &= 6\cdot45-6 \cdot \frac{y}{6}&& \text{cancel out the denominators} \\[1 em]288 &= 270-y&& \text{ swap the left and right side of the equation } \\[1 em]270-y &= 288&& \text{ move 270 to the right } \\[1 em]-y &= 288-270&& \\[1 em]-y &= 18&& \text{switch signs on both sides} \\[1 em]y &= -18&& \\[1 em] \end{aligned} $$

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