9. \(\sqrt{9a}+\sqrt{25a}-\sqrt{36a}\)

При \(a\ge 0\): \(\sqrt{9a}=3\sqrt a\), \(\sqrt{25a}=5\sqrt a\), \(\sqrt{36a}=6\sqrt a\).

\(3\sqrt a+5\sqrt a-6\sqrt a=2\sqrt a\).

Ответ: \(2\sqrt a\), при \(a\ge 0\).