$$ \begin{aligned} 42+90 &= 69x+90-79x&& \text{simplify left and right hand side} \\[1 em]132 &= -10x+90&& \text{ swap the left and right side of the equation } \\[1 em]-10x+90 &= 132&& \text{ move 90 to the right } \\[1 em]-10x &= 132-90&& \\[1 em]-10x &= 42&& \text{ divide both sides by $ -10 $ } \\[1 em]x &= -\frac{42}{10}&& \\[1 em]x &= -\frac{21}{5}&& \\[1 em] \end{aligned} $$

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