\[\boxed{\mathbf{609\ (609).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[= \sqrt{\left( \sqrt{a} + 6 \right)^{2}} - \sqrt{\left( \sqrt{a} - 6 \right)^{2}} =\]
\[= |\sqrt{a} + 6| - |\sqrt{a} - 6|\]
\[при\ a > 36:\]
\[\sqrt{a} + 6 - \sqrt{a} + 6 = 12.\]
\[при\ \ 0 \leq a \leq 36:\]
\[\sqrt{a} + 6 - 6 + \sqrt{a} = 2\sqrt{a}.\]