[tex] \frac{12}{ \sqrt{7} - \sqrt{3} } [/tex]

[tex] \frac{12}{ \sqrt{7} - \sqrt{3} } \times \frac{ \sqrt{7} + \sqrt{3} }{ \sqrt{7} + \sqrt{3} } [/tex]

[tex] \frac{12( \sqrt{7} + \sqrt{3} )}{ \sqrt{ {7}^{2} } - \sqrt{ {3}^{2} } } [/tex]

[tex] \frac{12 \sqrt{7} + 12 \sqrt{3} }{7 - 3} [/tex]

[tex] \frac{\cancel{12 {}}^{3} \sqrt{7} + \cancel{12 {}}^{3} \sqrt{3} }{\cancel4} [/tex]

[tex]\boxed{\bf\blue{3 \sqrt{7} + 3 \sqrt{3} }}[/tex]