[tex]\sqrt{39}\cdot\sqrt{27} =\\ \\ = \sqrt{3\cdot 13}\cdot\sqrt{3^2\cdot 3} = \\ \\ =\sqrt{3\cdot 13\cdot 3^2\cdot 3} = \\ \\ = \sqrt{3^2\cdot 3^2\cdot 13} = \\ \\ =\sqrt{3^2}\cdot \sqrt{3^2}\cdot \sqrt{13} = \\ \\ = 3\cdot 3\cdot \sqrt{13} = \\ \\ = 9\sqrt{13}[/tex]
