\[\sqrt{9 + 4\sqrt{2}} =\]
\[= \sqrt{8 + 1 + 2 \bullet 2\sqrt{2} \bullet 1} =\]
\[= \sqrt{\left( 2\sqrt{2} + 1 \right)^{2}} = \left| 2\sqrt{2} + 1 \right| =\]
\[= 2\sqrt{2} + 1.\]