\[\left( 2\sqrt{6} - 3\sqrt{7} \right)^{2} =\]
\(= 4 \cdot 6 - 12\sqrt{42} + 9 \cdot 7 =\)
\[= 24 - 12\sqrt{42} + 63 =\]
\[= 87 - 12\sqrt{42}\]