a) \(5x\left(x-4\right)-x^2+16=0\)⇔\(5x^2-20x-x^2+16=0\)
⇔\(4x^2-20x+16=0\)⇔\(\left(2x-5\right)^2-9=0\)
⇔\(\left(2x-8\right)\left(2x-2\right)=0\)⇔\(\left[{}\begin{matrix}x=1\\x=4\end{matrix}\right.\)
b) \(x^2-4x+3=0\)⇔\(x^2-x-3x+3=0\)
⇔\(x\left(x-1\right)-3\left(x-1\right)=0\)⇔\(\left(x-1\right)\left(x-3\right)=0\)
⇔\(\left[{}\begin{matrix}x=1\\x=3\end{matrix}\right.\)