\[\begin{split} &\lim_{n \to \infty} n\left(\sqrt{7n^2+3}-\sqrt{7n^2-3}\right)=\\[16pt] &=\lim_{n \to \infty} \frac{n\left(7n^2+3-(7n^2-3)\right)}{\left(\sqrt{7n^2+3}+\sqrt{7n^2-3}\right)}=\\[16pt] &=\lim_{n \to \infty} \frac{6n}{\left(\sqrt{7n^2+3}+\sqrt{7n^2-3}\right)}\frac{:n}{:n}=\\[16pt] &=\lim_{n \to \infty} \dfrac{\dfrac{6n}{n}}{\sqrt{\dfrac{7n^2}{n^2}+\dfrac{3}{n^2}}+\sqrt{\dfrac{7n^2}{n^2}-\dfrac{3}{n^2}}}=\\[16pt] &=\lim_{n \to \infty} \dfrac{6}{\sqrt{7+\dfrac{3}{n^2}}+\sqrt{7-\dfrac{3}{n^2}}}=\\[16pt] &=\frac{6}{2\sqrt{7}}=\frac{3}{\sqrt{7}}=\frac{3\sqrt{7}}{7} \end{split}\]