Bonsoir,
1)
[tex]A=(2x-1)(3x-8)-2x+1\\=(2x-1)(3x-8)-(2x-1)\\=(2x-1)(3x-8-1)\\=(2x-1)(3x-9)[/tex]
2)
[tex]2x^2-7x+3=0\\2x^2-x-6x+3=0\\x(2x-1)-3(2x-1)=0\\(2x-1)(x-3=0\\2x-1=0\quad ou\quad x-3=0\\2x=1\quad ou\quad x=3\\\\x=\dfrac{1}{2}\quad ou\quad x=3\\\\\boxed{S=\left\{\dfrac{1}{2};3\right\}}[/tex]