$$ \begin{aligned} 9 &= 7y+8\cdot(-18-2y)&& \text{simplify right side} \\[1 em]9 &= 7y-144-16y&& \\[1 em]9 &= -9y-144&& \text{ swap the left and right side of the equation } \\[1 em]-9y-144 &= 9&& \text{ move -144 to the right } \\[1 em]-9y &= 9+144&& \\[1 em]-9y &= 153&& \text{ divide both sides by $ -9 $ } \\[1 em]y &= -\frac{153}{9}&& \\[1 em]y &= -17&& \\[1 em] \end{aligned} $$

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