a: \(x^2-6x+5=0\)
\(\Leftrightarrow\left(x-1\right)\left(x-5\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=1\\x=5\end{matrix}\right.\)
b: \(x\left(x+5\right)+x\left(x+15\right)=0\)
\(\Leftrightarrow x\left(2x+20\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-10\end{matrix}\right.\)