$\displaystyle \begin{array}{{>{\displaystyle}l}} ( 6x+1)( 3x-2) +x( 7-6x) =-21\\ 18x^{2} -12x+3x-2+7x-6x^{2} +21=0\\ 12x^{2} -2x+19=0\\ 11x^{2} +\left( x^{2} -2x+1\right) +19=0\ \\ 11x^{2} +19+( x-1)^{2} =0\ \\ Vì\ 11x^{2} +( x-1)^{2} \geqslant 0\ với\ mõi\ \\ \rightarrow 11x^{2} +19+( x-1)^{2} =0\ ( vô\ lí\ ) \ \\ \Longrightarrow Vô\ nghiệm\ \ \\ \\ \\ \end{array}$
