\(a,m=2\\ PT\left(\text{*}\right)\Leftrightarrow x^2+4x-12=0\\ \Leftrightarrow x^2-2x+6x-12=0\\ \Leftrightarrow\left(x-2\right)\left(x+6\right)=0\Leftrightarrow\left[{}\begin{matrix}x=2\\x=-6\end{matrix}\right.\\ b,\text{Vi-ét: }\left\{{}\begin{matrix}x_1+x_2=-4\left(m-1\right)=4-4m\\x_1x_2=-12\end{matrix}\right.\\ \text{Ta có }4=\left(x_1+x_2-x_1x_2-8\right)^2\\ \Leftrightarrow\left(4-4m+12-8\right)^2=4\\ \Leftrightarrow\left(8-4m\right)^2=4\\ \Leftrightarrow\left(4-2m\right)^2=1\\ \Leftrightarrow\left[{}\begin{matrix}4-2m=1\\2m-4=1\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}m=\dfrac{3}{2}\\m=\dfrac{5}{2}\end{matrix}\right.\)
