$$x\cdot2-10x+21 = 0$$
Answer
$$ x = \frac{21}{8} $$
Explanation
$$ \begin{aligned} x\cdot2-10x+21 &= 0&& \text{simplify left side} \\[1 em]x\cdot2-10x+21 &= 0&& \\[1 em]-8x+21 &= 0&& \text{ move the constants to the right } \\[1 em]-8x &= -21&& \text{ divide both sides by $ -8 $ } \\[1 em]x &= \frac{-21}{-8}&& \\[1 em]x &= \frac{21}{8}&& \\[1 em] \end{aligned} $$
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