\(\left(6x-4\right)\left(2x-7\right)+\left(3x+5\right)\left(x-4x\right)=-31\\ \Leftrightarrow12x^2-50x+28+3x^2-12x^2+5x-20x+31=0\)
\(\Leftrightarrow3x^2-65x+59=0\)
\(\Leftrightarrow x^2-\dfrac{65}{3}x+\dfrac{59}{3}=0\\ \Leftrightarrow x^2-2.\dfrac{65}{6}x+\left(\dfrac{65}{6}\right)^2=-\dfrac{59}{3}+\left(\dfrac{65}{6}\right)^2\)
\(\Leftrightarrow\left(x-\dfrac{65}{6}\right)^2=\dfrac{3517}{36}\)
\(\Rightarrow\left\{{}\begin{matrix}x-\dfrac{65}{6}=\dfrac{\sqrt{3517}}{6}\\\dfrac{65}{6}-x=\dfrac{\sqrt{3517}}{6}\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{\sqrt{3517}+65}{6}\\x=\dfrac{-\sqrt{3517}+65}{6}\end{matrix}\right.\)
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