$$y = 120-2\cdot(150-2y)$$

$$ \begin{aligned} y &= 120-2\cdot(150-2y)&& \text{simplify right side} \\[1 em]y &= 120-(300-4y)&& \\[1 em]y &= 120-300+4y&& \\[1 em]y &= 4y-180&& \text{move the $ \color{blue}{ 4y } $ to the left side} \\[1 em]y-4y &= -180&& \text{simplify left side} \\[1 em]-3y &= -180&& \text{ divide both sides by $ -3 $ } \\[1 em]y &= \frac{-180}{-3}&& \\[1 em]y &= 60&& \\[1 em] \end{aligned} $$

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