\[\left( \sqrt{75} - \sqrt{12} \right)\sqrt{3} = \sqrt{75 \cdot 3} - \sqrt{12 \cdot 3} =\]
\[= \sqrt{25 \cdot 3 \cdot 3} - \sqrt{36} = 5 \cdot 3 - 6 =\]
\[= 15 - 6 = 9\]