\(\displaystyle{x}^{{2}}-{6}{x}+{9}={20}\) factor the left side
\(\displaystyle{\left({x}-{3}\right)}^{{2}}={20}\) now take the square root of each side
\(\displaystyle{x}-{3}=+{\quad\text{or}\quad}-\sqrt{{{20}}}\)
\(\displaystyle{x}=\sqrt{{{20}}}+{3}{\quad\text{or}\quad}{x}=-\sqrt{{{20}}}-{3}\)
\(\displaystyle{x}={2}\sqrt{{{5}}}+{3}{\quad\text{or}\quad}{x}=-{2}\sqrt{{{5}}}-{3}\)