\[y = \sqrt{x^{2} - 14x + 40}\]
\[x^{2} - 14x + 40 \geq 0\]
\[D = 49 - 40 = 9\]
\[x_{1} = 7 + 3 = 10;\ \ \ x_{2} = \pm 3 = 4\]
\[(x - 4)(x - 10) \geq 0\]
\[D(y) = ( - \infty;4\rbrack \cup \lbrack 10; + \infty).\]