`(5x+3)^2-(2x-7)^2=x-40`
`<=>(5x+3+2x-7)(5x+3-2x+7)-x+40=0`
`<=>(7x-4)(3x+10)-x+40=0`
`<=>21x^2+70x-12x-40-x+40=0`
`<=>21x^2+57x=0`
`<=>x(21x+57)=0`
`<=>` $\left[\begin{matrix} x=0\\ 21x+57=0\end{matrix}\right.$
`<=>` $\left[\begin{matrix} x=0\\ x=-\dfrac{57}{21}\end{matrix}\right.$
\(\left(5x+3\right)^2-\left(2x-7\right)^2=x-40\)
\(\Leftrightarrow25x^2+30x+9-\left(4x^2-28x+49\right)=x-40\)
\(\Leftrightarrow25x^2+30x+9-4x^2+28x-49-x+40=0\)
\(\Leftrightarrow21x^2-57x=0\)
\(\Leftrightarrow3x\left(7x-19\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=\dfrac{19}{7}\end{matrix}\right.\)
