\[f(x) = \sqrt{x - 3} - \sqrt{6 - x}\text{\ \ }\]
\[\left\{ \begin{matrix} x - 3 \geq 0 \\ 6 - x \geq 0 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ }\left\{ \begin{matrix} x \geq 3 \\ x \leq 6 \\ \end{matrix} \right.\ \]
\[\Longrightarrow D(f) = \lbrack 3;6\rbrack.\]