\[y^{2} = \left( \sqrt{7} - \sqrt{3} \right)\left( \sqrt{7} + \sqrt{3} \right)\]
\[y^{2} = \left( \sqrt{7} \right)^{2} - \left( \sqrt{3} \right)^{2}\]
\[y^{2} = 7 - 3\]
\[y^{2} = 4\]
\[y = \pm 2.\]