Solution
We can do the following:
[tex]x^2-12x+(\frac{12}{2})^2=-23+(\frac{12}{2})^2[/tex]And if we simplify we got.
[tex]x^2-12x+36=-23+36[/tex][tex](x-6)^2=13[/tex]Taking square root we got:
[tex]x-6=\pm\sqrt[]{13}[/tex]Then the two solutions are:
[tex]x_1=-\sqrt[]{13}+6,x_2=\sqrt[]{13}+6[/tex]