$$ \begin{aligned} 25(a+70) &= 15(a-50)&& \text{simplify left and right hand side} \\[1 em]25a+1750 &= 15a-750&& \text{move the $ \color{blue}{ 15a } $ to the left side and $ \color{blue}{ 1750 }$ to the right} \\[1 em]25a-15a &= -750-1750&& \text{simplify left and right hand side} \\[1 em]10a &= -2500&& \text{ divide both sides by $ 10 $ } \\[1 em]a &= -\frac{2500}{10}&& \\[1 em]a &= -250&& \\[1 em] \end{aligned} $$

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