\[\left( 7\sqrt{2} - 3\sqrt{3} \right)^{2} = \left( 7\sqrt{2} \right)^{2} - \left( 3\sqrt{3} \right)^{2} =\]
\(= 49 \cdot 2 - 9 \cdot 3 = 98 - 27 = 71\)