D\(=\left(3\sqrt{2}+\sqrt{6}\right)\sqrt{6-3\sqrt{3}}\)
D\(=3\sqrt{2}.\sqrt{6-3\sqrt{3}}+\sqrt{6}.\sqrt{6-3\sqrt{3}}\)
D\(=3\sqrt{12-6\sqrt{3}}+\sqrt{36-18\sqrt{3}}\)
D\(=3\sqrt{\left(3-\sqrt{3}\right)^2}+\sqrt{\left(3\sqrt{3}-3\right)^2}\)
D\(=3\left(3-\sqrt{3}\right)+3\sqrt{3}-3\)
D\(=9-3\sqrt{3}+3\sqrt{3}-3\)
\(D=9-3=6\)