\(a,\left(x-3\right)^2-x\left(x-4\right)=5\\ \Rightarrow x^2-6x+9-x^2+4x=5\\ \Rightarrow-2x=-4\\ \Rightarrow x=2\\ b,x\left(x-6\right)+2x-12=0\\ \Rightarrow x\left(x-6\right)+2\left(x-6\right)=0\\ \Rightarrow\left(x+2\right)\left(x-6\right)=0\\ \Rightarrow\left[{}\begin{matrix}x=-2\\x=6\end{matrix}\right.\)