\[\left( \sqrt{63} - \sqrt{28} \right) \cdot \sqrt{7} =\]
\[= \sqrt{63} \cdot \sqrt{7} - \sqrt{28} \cdot \sqrt{7} =\]
\[= \sqrt{63 \cdot 7} - \sqrt{28 \cdot 7} =\]
\[= \sqrt{9 \cdot 7 \cdot 7} - \sqrt{4 \cdot 7 \cdot 7} =\]
\[= 3 \cdot 7 - 2 \cdot 7 = 21 - 14 = 7\]