Rút gọn biểu thức

$$\begin{gathered} \mathrm{Rút} \mathrm{gọn} \mathrm{biểu} \mathrm{thức}: \\ \mathrm{a}, \mathrm{A}=\sqrt{6+3\sqrt{3}}+\sqrt{6-3\sqrt{3}} \\ \mathrm{b}, \mathrm{B}=\frac{(5+2\sqrt{6})(49-20\sqrt{6})\sqrt{5-2\sqrt{6}}}{9\sqrt{3}-11\sqrt{2}} \end{gathered}$$