\[\sqrt{x^{2} - 8x - 23} = 5\]
\[t = x^{2} - 8x - 23\]
\[\sqrt{t} = 5\]
\[t = 25 \Longrightarrow x^{2} - 8x - 23 = 25\]
\[x^{2} - 8x - 23 - 25 = 0\]
\[x^{2} - 8x - 48 = 0\]
\[x_{1} = 12;\ \ x_{2} = - 4\]
\[Ответ:12;\ - 4.\ \]