\(b,\frac{9-a}{\sqrt{a}+3}-\frac{9-6\sqrt{a}+a}{\sqrt{a}-3}-6\)
\(=\frac{\left(3-\sqrt{a}\right)\left(3+\sqrt{a}\right)}{\sqrt{a}+3}-\frac{\left(\sqrt{a}-3\right)^2}{\sqrt{a}-3}-6\)
\(=3-\sqrt{a}-\left(\sqrt{a}-3\right)-6\)
\(=3-\sqrt{a}-\sqrt{a}+3-6\)
\(=-2\sqrt{a}\)