Answer
$3\sqrt{30}-3\sqrt{35}$
Work Step by Step
Using the Distributive Property which is given by $a(b+c)=ab+ac,$ the given expression is equivalent to
\begin{array}{l}\require{cancel}
3\sqrt{5}(\sqrt{6}-\sqrt{7})
\\\\=
3\sqrt{5}(\sqrt{6})+3\sqrt{5}(-\sqrt{7})
.\end{array}
Using the Product Rule of radicals which is given by $\sqrt[m]{x}\cdot\sqrt[m]{y}=\sqrt[m]{xy},$ the expression above is equivalent to\begin{array}{l}\require{cancel}
3\sqrt{5(6)}-3\sqrt{5(7)}
\\\\=
3\sqrt{30}-3\sqrt{35}
.\end{array}