\[\left( 2\sqrt{7} + 3 \right)^{2} = \left( 2\sqrt{7} \right)^{2} + 2 \cdot 6\sqrt{7} + 3^{2} =\]
\[= 4 \cdot 7 + 12\sqrt{7} + 9 =\]
\[= 28 + 12\sqrt{7} + 9 = 37 + 12\sqrt{7}\]