\[\sqrt{\left( x^{2} + 6x \right)^{- 1}} = \sqrt{\frac{1}{x^{2} + 6x}}\]
\[x^{2} + 6x > 0\]
\[x(x + 6) > 0\]
\[x < - 6;\ \ x > 0.\]
\[D(f) = ( - \infty; - 6) \cup (0; + \infty).\]